As for what is considered a "legend" I really don't know, football is weird like that. It could be that you achieve this status by being an aggressive no-bullshit kinda guy (Vinnie Jones at Wimbledon), a world class player who stays at the same club his entire life (Ryan Giggs @ Man Utd) or just being an insanely prolific striker who scores for fun .
On this topic though, various newspapers\magazines\sites periodically publish "worst transfers" lists which are basically filler (everyone who cares already knows who will be on the lists), but which could be surprisingly enlightening for someone who's not familiar with football's transfer scene. This one's a bit advert-y but you'll get the gist: http://bleacherreport.com/articles/1610214-13-stupidest-tran...
Unknown players will be cheaper on both, but even if you picked them up on a one year contract and they immediately proved that their next contract should be highly paid, you've still saved a potentially huge amount on buying them before they were worth a huge signing fee.
Gradually salary will catch up based on how well they play and based on the market, and the team that found this cheap player can either stop considering him a cheap player (but still be glad he came in cheaply), or can sell him on for a profit.
The top 10 salaries in the premier league range from ~£10m/year to ~£15m/year, so consider that buying a top player and keeping them for 2 years can often cost £20m-£80m. If you can buy a player for cheap and pay him not a huge amount, you can afford to spend a few years turning him into a star player and still come out way ahead even if in those few years he wasn't giving you value other than turning into a better player.
How long does it take for a player to become legendary? "Legendary" would normally be reserved either for the very best of the best, or on a team level based on both quality but also loyalty - as such you'd rarely see someone be called a legend before spending quite a few years at a club. But a player can go from relatively unknown to "shit he's really good" in as little as a season potentially - maybe he was playing in a league that doesn't get as much attention from countries with the bigger leagues, maybe he needed better team-mates around him to bring out his best, maybe a manager or coach helped him change his playing style very quickly, etc.
Clubs 'sell' players who are already contracted to them, to other clubs, in order to have the contract nullified. The price can be tens of millions of euro. The buying club will tie down the new player to a long contract as part of the purchase, perhaps 4 or 5 years.
Wages are low for young players, but will be increased rapidly if the player emerges as a real talent that other clubs would want, which gives the club the chance to extend the contract through renegotiation. A longer contract for a highly rated young player would mean higher wages also as part of the bargain, although contracts can be renegotiated and extended with higher wages at any time, if the club views the player as an asset that they wish to keep.
When they extend that contract it generally means that another club who tries to buy will have to pay more, and usually wages are at least matched and often increased by the buying club.
Sorry that's not a succinct answer.
There is occasional talk about doing away with the system, as it goes against a lot of employee rights principles.
Take the example of James Rodríguez[1], a young Colombian player. His "value" suddenly skyrocketed[2] when he scored this wonder goal[3] in the 2014 World cup. Next thing, Real Madrid snapped him up for €80million, the decision at least partially influenced by Real's "Galaticos"[4] transfer policy, and the fact that Real can increase their commercial influence in a Spanish speaking country because Rodríguez is Colombian.
Other factors that influence a players value:
- time left on contract (longer time means more money)
- "marketability" e.g. player's ability to sell lots of club merchandise, and improve the club's brand
- perceived desperation of the buying/selling club (do they have a desperate need for a top class striker to make up for a lack of goals?)
- if there is a bidding war between clubs (e.g. young French striker Antony Martial was subject to a bidding war between Manchester United and Chelsea, dramatically driving up his value.)
- perceived transfer war chest of the buying club
- player's age
- if the player came off the back of a successful international tournament, which dramatically (sometimes artificially) showcases their skill. Sometimes teams will buy a player ahead of the tournament to prevent a later bidding war.
- skill of the player's agent (e.g. Raheem Sterling to Man City)
- then players skill and so on...
Every deal is a virtual once-off, it's always a bargain in hindsight[5] (whether you pay little or big money or not) and the player exceeds whatever the initial expectations were.
[1] https://en.wikipedia.org/wiki/James_Rodr%C3%ADguez
[2] http://www.transfermarkt.com/james-rodriguez/profil/spieler/...
[3] https://www.youtube.com/watch?v=GESyL3MkgNU
[4] https://en.wikipedia.org/wiki/Gal%C3%A1cticos
[5] http://www.independent.co.uk/sport/football/premier-league/a...
- Country of origin (home grown player quotas, visas, participation in national team (especially African cup of nations that occurs mid season).
- Attractiveness (something Madrid apparently value as part of their brand, and marketability)
- Mentality (Mario...)
- Position (typically higher the further up the pitch they play)
- Previous contract value. A number of players signed by big clubs then released are on high wages, and expect they to be met.
- time left on contract (shorter time means higher wage demands)