That depends on what kind of a spender her DH is. If she's the money-manager of the family, saving on her airfare but then letting him spend unchaperoned might end up costing more. Snacks and lunches at SeaWorld, etc.
Last year we went to FL for 10 days, and rented a car, and ooh - I just looked it up, it was $385 for 10 days. You got a good deal.
This is a useless post - I thought it was much less. We did wait until the last min. to reserve it and the cost kept going down each day or so as we got closer.
Good luck. I'd probably go for it by the way. We are in a similar predicament regarding a wedding of some very close friends. It's out of state, and with airfare, cabs to and from the airport, tux rental, gift, etc, we will be out of pocket over $1,000. And that's with staying at a rental house with family and leaving the kids here with my in-laws. This is a destination wedding for everyone, and it makes me MAD that they expect everyone to just suck it up and go along with it. But anyhow....we can't really afford it either but are putting the relationship first. (Groom is dd's godfather, etc)