Well we've experience this...our vet calls it "enlarged uterus".
Before the heat, their uterus is small and easier(?) to remove. When they are older, it's surrounded by fat which can be dangerous apparently, and has enlarged quite a bit.
Most of our vets charge 1 fee regardless of age, etc.
However, the one vet will charge, like, $60 or so, but it goes up to just over $100 for a post-heat spay, any time post 1st heat that is. So in reality he's no cheaper since most of the girls we get in rescue have gone through at least one heat, and that vet in particular hasn't done puppy- alters for us.
It kind of sucks.