Transfermarkt is a pretty decent relative guide if you ask me but the market is so weird that they'll never be accurate as an actual guide as to a players real value. I think it was Dodge who used to say that we can have all the opinions we like but a player is only worth what a club is willing to pay for him. Assigning value now is so difficult as the market is so volatile. Even more so now with contract lengths & amoritization and PSR rules (spending constraints, homegrown exemptions, etc).