The difference is that Rashford is on a lot higher salary, last I saw it was 4 times what Dan James was on, so if PSG match Rashford's wages, he signs a 4 year deal and sees out the deal, then he is on an additional £40 million on top of the transfer fee, whereas for the same salary Dan James is only worth £10 million on top of the transfer fee.
I agree £25m would be cheap, but considering his form and salary, last year of his contract the chance to move him on, I think £40-50m in a fantastic offer and £60m is an unbelievable offer.