We paid something toward our insulation as well. I think that's pretty standard.
From what I've seen, payment varies quite a bit from scheme to scheme, depending on the amount of grant money they have to work with, and who's supplied that money.
The first team that was supposed to do ours was going to charge around £200. The second team, who actually carried out the work, didn't charge anything. Though, of course, the house is owned by a pensioner, although no one is on any benefits. It's probably much harder to get it for free if you're under 60, not disabled or otherwise ill, and not on benefits.