Transfermarkt valuations are mostly bollocks RER and you should pay little attention to them.
Carrabuh I've explained this before. You always react in such a close-minded fashion when you're ignorant about things.
The transfermarkt valuations are integral to clubs' balance sheets - it's how they calculate their total assets. It's absolutely imperative that you have an asset valuation so that you can offset the amortisation against your profit. It's exactly the same, if you run a business, as valuing your computers and then calculating the depreciation over time (because eventually they become worthless and need replacing).
Transfermarkt is a pro valuation company. Those ones online are rough (almost certainly computer-generated) valuations based on a few key variables.
Every time a club gets a player, they will hire a company like transfermarkt to carry out a full valuation. This will be based on the key variables above, with a whole load more assessments to get a very accurate figure of a player's "value" as a "fixed asset". It is absolutely imperative that an assessment like this is carried out because everything is counted down to the last penny.
There is absolutely nothing bollocks about them.
As for their utility for transfer prices, as others have already explained, it's a bit of a guessing game. But a player will rarely go for more than about 3.5-4 times asset valuation and that would only be if a club is desperate for them and they are on a long contract. I.e. not Brady.
It's obvious that the valuations are pretty much bang on, though, because if a club is keen to sell a player the price is almost always within a couple of hundred thousand (which is the margin of error when the more accurate valuation is carried out). Take a look at Tom Ince, as the perfect example of a player going for bang on his asset value because a club was keen to sell...
The other reason the valuations are essential, is not for us but from a legal perspective. This allows clubs to ensure their directors/managers etc aren't doing anything dodgy.
If you don't believe any of that, read up on the Jean Tigana case. It's all there. This valuation malarky has been going on for years and it gets ever more complicated and important.
Honestly, I think some of you see the Premier League as just an expensive version of village cricket. Open your eyes, it's big business. I would go so far as to say that if the above explanation is nothing less than slaphead style screamingly obvious once it's been explained why, then you really are more dogmatically ignorant than I thought.