I think the formula consists of players quality (what better players or weaker players are worth), age, potential. Ofcourse recent sales and amount of similar players in the game or on the market also play a role. I believe BB-s have somekind of player valueing system or else they would not be able to evaluate player salarys during off season etc.
I agree with most of that, however there are other things that influence how much a player is worth such as a player's nationality, or whether he is a NT player. Also a players game shape might be reason to pay extra because you want to use him in a game straight away.
All players have a price range of what they are "worth" but this price range cannot be determined and is constantly fluctuating. I believe the price range to be the cheapest price they would sell for to the maximum price they would sell for.
You said that the BBs have a way of valueing players based on salary, but this isn't entirely correct. A player with 15k salary isn't necessarily more valuable than a player with 10k salary. The 15k player might have mediocre on all stats with tremendous jumpshot. The 10k player might have prominent OD, prominent JS, strong JR and Strong Handling and Driving with Respectable Passing. Clearly the 10k player is worth more.