It's not fixed, but there is a formula that takes a load of things into account from how long you have been at the club to, I believe, number of 1st team appearances and the category of the club trying to sign you. Leeds are a category 1 club so have to pay more.
So if you are mince, never made a 1st team appearance, only been at the club a couple of years and being signed by a lesser team, then the development fee would be much reduced.