Surely this could have gone into the existing thread discussing the point? The very one you refer to.
IMO, £6m is about right for a proven quality striker able to play in the NPC immediately, plus the Prem should we win promotion (although, of course, that's not yet proven). If the valuations of JRod, Maynard, Sharp et el of anything up to or over £4m are deemed good value, £6m for somebody who is regarded a better player seems good value to me.
It's also worth considering that in the case that has inspired this debate, we're trying to prise a player from a club who don't want to sell him. It's not like the player has said he wants out, or wants to play at a higher level so needs to leave, as is the case with some other targets. Celtic want to keep Hooper and would be delighted to do so, which means the price will rise if we're serious about buying him.