As JohnsonWes stated below, it depends on the amount and type of filler that's used. Restylane and Juvederm fillers cost anywhere between $500 and $1,200 per 1ml syringe. For full lips, less than 1ml should be needed.
It depends on how much filler is going to be used. Unless you asked them for a specific amount (i.e. more than 1 syringe), I would say that $800 is kind of steep. Usually the $500-600 range is standard.