you may try calling other dealers for a quote. there is some variation in the prices dealers charge for parts, so you may find it cheaper from a different dealer even when factoring in shipping.
as far as that $100 labor, tell them you'll do it yourself! you've already taken the mirror off--putting one back on is not that much more difficult, especially if you take a look at one of the several online guides (e.g. evilution.co.uk).