Sadly when has it ever been as simple as comparing two players and going 'hmm, he's easily worth 1.45x a Carroll"?
We have Nasri for example going for possibly less than £10m. :/
Nasri wouldn't be worth 20mil if he was on a long contract, he's got a year left on his contract, so why should he be worth more than 10mil?
Carroll's price was dictated by Torres's price, and the time limits imposed on the deal, Liverpool showed their interest, there was little to no time left, they didn't have time to conclude a deal for another striker, that all massively effected the price. Likewise Carroll had just signed a new long term contract and loved being in Newcastle.
Modric, while having done well at Spurs, is someone higher up clubs see as a guy whose tasted the champs league, wants the champs league and will want to move onwards and upwards from Spurs, again effecting his value.
If Spurs were in the CL again, then Modric might be worth more, but Spurs not being in the CL mean Utd can offer something Spurs can't, and that also means Modric could look to leave/ask for a transfer, which effects his value.
When everyone randomly ignores things like time left on contract, what the various clubs can offer, and the likelyhood of the player to want to stay, sure all transfer fee's seem absolutely illogical, when you factor in all the pretty obvious things, transfer fee's aren't particularly hard to work out.
Bent's one of the best strikers in the league, he's proven over many seasons and is up their with the best 3-4 in the league in terms of goals scored, ultimately what they are being bought for, 24mil isn't even remotely expensive for him. Just the random reputation England players get, Theo absolutely beloved at Arsenal for being English yet shouldn't play anywhere above the league one. Young barely wanted at Liverpool or Utd, yet deserves to be first team for England without a shadow of a doubt. Bent, clearly the most consistant England striker in the league for years yet zero respect.
I honestly can't explain it, but its got a huge effect on the value people perceive for these guys. People saying Young's never worth 15mil +, Bent at 24mil is a joke, if they were Brazillian, French, Spanish, people would be begging for their club to match their 30mil + valuation.