How do Portugal's top clubs continue to win big in the transfer market?

And another big deal completed -- in this case by Benfica, selling Goncalo Guedes to Paris Saint-Germain for €30 million plus up to €7m on a future sell-on fee.

How is it that one of Europe's lower-profile leagues plays such a prominent role in the global transfer market and commands such high fees for its best players?

"How is it possible? Portuguese football is at the level of the best in Europe," says Portuguese agent Nuno Correia, owner of NCfoot, a Portuguese agency. "Proof of this is we have the best player in the world, Portugal are European Champions, we have the best coach in the world and we have the most important agent in the world."

Like all good business models, there is no simple single reason but rather several contributing factors that have converged to make Portugal the perfect stage to attract, develop and sell footballers of above-average calibre.

The raw material

There's a reason you've never heard of a famous Portuguese-born tennis player, golfer or athlete. The beautiful game is everything in this football-crazed country, with blanket media coverage, including three daily sports papers. Together with the stark lack of alternatives in one of Europe's weakest economies -- where the previous Prime Minister Pedro Passos Coelho infamously invited the country's young to emigrate to better themselves -- football is seen by many adolescents, especially from underprivileged backgrounds, as the only route to fame and fortune.

In contrast, Portugal is an enticing prospect for talented young foreigners as they seek to make their way up the football food chain. For the best of them who are scouted by Benfica, Porto and Sporting, clubs where Champions League football is virtually guaranteed every year and they benefit from top-level coaching, it is the perfect chance to enhance their skills, put themselves in the shop window and ultimately earn a move to the biggest clubs in Europe.

"Portuguese clubs don't have much money but thanks to good scouting they can attract good players, with a lot of potential at a low cost, who look at Portugal as a launching pad to get to the biggest leagues in the world: Spain, England, Italy, Germany," says Correia.

"Portugal has a competitive league with good coaches and good working conditions. Then for Brazilian and South American players there's the question of the climate, the hospitable nature of the country, the culture -- and the language in the case of Brazil -- which makes it easier for them to adapt."

In turn, the clubs reap the rewards on the pitch -- and subsequently, in their bank accounts. It's a win-win situation.

Lisandro Lopez, Radamel Falcao and Hulk are just three examples of the "buy low, sell high" policy that was turned into a fine art form by Porto, but Benfica (Angel Di Maria and David Luiz, among many others) and Sporting (Islam Slimani) have since caught on.

Paradoxically, the fact that Portugal lags behind its European counterparts in terms of wealth is the very reason many of the world's brightest young football talents, whether Portuguese-born or foreign, are here in the first place.

"The European giants do not go for players who need developing," says Correia. "They wait for them to prove their worth. What those clubs want is 'ready-made' players who arrive and deliver straight away. They don't want players they have to work on. Only when they prove they are ready for the highest level do the giants come for them."

Polishing the product

No matter how talented the raw material, without the right coaching and a propitious learning environment, it would be impossible for players to fetch such large transfer fees. With that in mind, the success of Carlos Queiroz's work in nurturing Portugal's first golden generation in the late 1980s and early 90s proved a watershed moment for coaching in the country.

Football club presidents put more faith in national coaches; several of Portugal's universities began running degrees in the sports performance and coaching fields, while the Portuguese Football Federation invested heavily in setting up courses and organising workshops and seminars to hone the talents of up-and-coming treinadores.

The spectacular success of Jose Mourinho and Andre Villas-Boas further consolidated the profession. It also saw studious and well-prepared coaches became favoured over former players, especially when it came to appointing youth coaches, and the results speak for themselves. While Euro 2016 was the first major triumph at senior level, Portugal has achieved impressive results at international youth tournaments throughout the last three decades.

It's not just the youngsters in the academies who benefit from top-class coaching.

"Jorge Jesus is a world-class coach, and I'm not exaggerating," Islam Slimani told France Football last year. "Tactically, he's a genius. He makes us progress every day." With a big emphasis on tactical discipline, a spell in Portugal often transforms untamed talent into the finished article.

The sell

The success at the business end of the transfers is inextricably linked to one man: Portuguese "super-agent" Jorge Mendes. The former nightclub owner seized his opportunity as football transformed into a global multibillion-dollar business, forging mutually beneficial relationships with players, coaches, chairmen and presidents all over the world.

"Jorge Mendes is the best agent I dealt with, without a doubt," wrote Sir Alex Ferguson in his autobiography. "He was responsible, looked after his players and was very fair with clubs."

Mendes' contacts in the game are unparalleled; his ability to find clubs willing to pay big for Portuguese talent is also without equal. His most famous clients are Cristiano Ronaldo and Mourinho, but his Gestifute stable oversees the careers of hundreds of players and managers, and he has been responsible for the vast majority of the major sales of Portugal players over the past two decades.

"Today he is our partner and I have a completely open relationship with [Mendes]," Benfica president Luis Filipe Vieira told Portuguese TV channel TVI in September. "We have a business mentality." In addition to his close ties to Benfica, Mendes is usually heavily involved in Porto's transfers.

The election at Sporting of uncompromising president Bruno de Carvalho, a fierce critic of the non-transparent role agents and investment funds play in football, brought an end to Mendes' dealings with the Lions but so far, it hasn't hindered Sporting's ability to participate in the Portuguese transfer bonanza.

Carvalho initially attracted mockery for inserting unrealistically inflated release clauses into every new signing Sporting made, but the policy has now become common practice across the board. Last summer saw the club's two record sales as Joao Mario and Slimani brought in €45m and €35m respectively following their transfers to Inter Milan and Leicester.

"It's not difficult to sell Portuguese players because we are talking about high-quality footballers," explains Correia. "They may be expensive in terms of the transfer fee but not in terms of wages."

With everything in place to find and attract talent, nurture it and sell it, there's no reason to expect the pounds and euros from abroad will stop pouring into the coffers of Portuguese clubs anytime soon.

The bottom line is simple economics and a question of supply and demand. Or as Correia puts it: "Portuguese clubs have to sell, and the big European clubs come here and can get technically and tactically proficient players who don't exist in their countries."