Quite a lot of US schools charge $250 for writing out the I-20.
Actually it's not so much that they charge $250 for writing out the I-20 but they charge $250 as an advance entry fee. This is then deducted from the total package price when you eventually show up at the school. And the package includes the I-20.
I suppose they've got to put some form of deterrent into place to avoid 100s of bogus enrollments. $250 seems a bit steep to me but since you'll get it back eventually, I don't see the problem.