229 per week for a minivan (which is the rate from Dollar) is a pretty good rate for a minivan especially considering it is over a holiday weekend. Usually 199 is about as low as you can find them in Orlando unless you have a coupon or a special discount. The taxes in Florida are pretty high and that does take that 349.00 rate up a pretty good chunk.
Unfortunately Minivans in Orlando are a pretty high demand item and the agencies realize that. Fullsizes are always cheaper because it is easier for people to shop those. Many people who want a fullsize can fit into a midsize, etc. But if you need 6 or 7 seats in a car you don't have a lot of choice. Supply and demand at work. Good luck though and keep shopping, you may find one lower, but if you need a minivan sized car, I would hold that rate for now, it really isn't too bad considering.
PS, it looks like we will be down there about the same time you are. We are driving so far though, unless I can find an exceptionally cheap airfare since there are 6 of us travelling.