There could be many factors, depending on which, the selling team quotes the price of a particular player.
First and foremost, a footballer will be judged on the primary aspect that is expected of him, his footballing skills. Skillful players are generally the ones who can change the game on their own or pull the strings and control the tempo with which the game is played at. Here, skill should not be confused with show-boating. The more skillful a player his, the higher will be his market value.
Nationality of players
A foreign recruit with higher skill might cost less than a local player with lesser skill. This is because of each club is expected to have a certain number of home-grown players in their squad. If these home-grown players are good enough to play regularly in the playing XI, then it is a bonus, and therefore the local players are higher valued in the country. But when sold to another club outside the country, the cost of the player can go down.
A player at the end of his career, no matter how skillful he is, will be lesser valued in the market. Clubs would not like to spend big on a player whose skills and speed are deteriorating and have few months/years of footballing left in them. Clubs looking for a short-term solution to provide a thrust in avoiding relegation or mid-table clubs that are looking for European places generally go for such players.
This is simple economics. The more the demand for a player, the costlier his value in the market. If more than one club is battling for a player, it gives an initiative to the selling club to hike the players selling price.
Integration into team
Players with very good record of discipline are looked at a safer bet by buying clubs as it is easier to integrate them into the existing team. Team-spirit is a vital thing to have on and off the pitch and it would make breathing easier for the clubs if a player is able to adjust into the team. Clubs also look at how a player, if bought, will fit into the teams current formation and strategy.
Each of the leagues has its own uniqueness. For example, the EPL is more physical where as the La Liga is more technical. The buying clubs would consider how a player from another league would adapt to its league before making a transfer bid for a player.
Buying a player does not always guarantee success due to various reasons. So, players with re-sale value tend to cost higher as the buying club can sell this player to another club in the future compared to players with lesser re-sale value (Age plays a predominant part).
Image for marketability
Players that are marketable come at a slightly higher price. These players are generally ones that can bring in a dramatic increase in shirt sales for the club. Also, the clubs look to increase the brand image of the club in that particular players home-country.
Current contract validity
A player that has a long term contract will be costlier than a player whose contract is due for expiry in the near future. If a player whose contract is set to end soon and he or the club is not keen on an extension, the current club will look to sell him on the cheap during the validity of the contract than lose him for free once the contract expires.
Other factors, such as the ones mentioned below, also impact the players price.
- A player being cup-tied (Domestic or European competitions) for the current season.
- Clubs with financial problems would look to sell players on the cheap to raise money.
- A club with a bigger financial profile will have to, at times, pay more to buy a player because of their image.
- A player's fitness may affect his transfer value.
A combination of the each of the above factors, and maybe more, go into what a player costs in the market during the transfer window.