Not only is it relative, but all down to market rate. People criticise the £40 million paid for Sterling, however looking at his ability and achievements at the age of 20, that's the Market rate. Martial is 19, has had very few first team appearances and hasn't scored many goals. He barely makes it in the Monaco team. When we signed Oxlade-Chamberlain a few years ago, the 16 million deal for an 18 year old with a few Ligue 1 appearances was getting a hammering!I don't think anyone cares if an opposition club get's raped over a fee. It's more funny than anything else.
But as regards your own club, it's all relative regardless of how much you have at your disposal or not. You still don't want to be paying anymore than you have to and maybe have a knock-on effect down the line with someone else.