Are these figures based on actual purchase price or some subjective evaluation of potential purchase price? If the latter, I think this kind of thing is almost worthless, since it is bound to be heavily biased by perceptions (e.g. 'little old Norwich' are going to get seriously undervalued).
In fact, the answer is, in an odd way, neither.
When any players get bought, due to clubs like Norwich being companies with shareholders and directors, each purchase is carefully assessed for financial reasons. Directors owe a duty to their shareholders so they have to do this to protect themselves (in the case of, e.g. Chelsea, this is less important because Abramovich might make the decision himself).
This means that there is a professional valuation of each player. These are extremely important if a club is ever to sue its directors (e.g. Jean Tigana). There are several different professional valuation companies. They take into account an enormous range of factors, including the player's physical and technical attributes and injury history, but also the club they are at (a bigger club simply adds value to a player), the league/competitions they play in, how they have performed against other teams, etc. Clubs use this valuation to help assess how much they are willing to pay. It is not the only deciding factor (contract might be nearly up, the manager might desperately want them, the player might be English, etc.) clubs use to decide whether to agree to a purchase, but it is a big one. More importantly, it is how the directors protect themselves if anyone was to review the purchase financially.
In essence, these figures are about as objective as you can get, but also they only bear relation to transfer prices when you take into account the other factors. E.g., you can fairly accurately predict that if a player is in the last year of his contract, he will go for around the price listed on the site. If his contract is longer than two or three years, the price will be somewhere between 2 and 3 times the value listed on the site. The multiple goes up when a club desperately wants to buy and the other doesn't want to sell (e.g. Andy Carroll to Liverpool was c. 4 times the value) or can plummet if the club is keen to sell or the player is keen to leave (e.g. Fulham paid about 0.5 of Berbatov's value despite a year left on his contract).
Transfermarkt GmbH & Co. is one of the main professional football player valuers, it is based in Germany. The valuations on their website are
not what would be given to a club who had commissioned a full report. However, they are a very reasonable guide. You will notice that transfermarkt also have very good info on the transfer price.
So what this gives us is a relatively objective and reasonably, if not entirely, accurate, guide as to the base value of each player. It does not tell you what price they will/should be bought and sold for, but it helps educate a guess. By base value, we could probably narrow this down to a combination of their technical and physical (including age) attributes and their footballing "achievements" (in the sense of how people generally perceive their career).