The rule with my Mum was that as long as we were studying, she'd support us, but if we finished/quit uni, then we had to pay board. I don't remember how much it was (early 90s), but it was more than fair. We also had jobs from when we were 15 & had to buy our own clothes, entertainment with that.
:yeahthat:

As others have said, it teaches/develops financial responsibility and means that it won't be such a rude shock when she does finally move out to pay rent/bills/food etc by herself.

If it's how you plan to do things in future then dicussing it now (not in a full on way but just dropping it into conversation at opportune times) might be a good idea so she knows what's coming and what will be expected and the reasons behind it. We always knew the 'rules' around this stuff in advance so it was just accepted and no big deal in our house.