I'm surprised to hear that your PTO has not created a budget line item for this and every other activity (event or otherwise) that the PTO sponsors.
I would recommend that you meet with the Executive Board and Principal in addition to the rest of your committee ASAP and discuss how reimbursements for this event will be handled. I would even recommend that you bring receipts or other items that show how much it takes to "put on" this event. Just because it's a fundraiser doesn't make it any less worthy of a budget.
If this is still not enough to warrant a budget, then I would inform everyone that since a budget has not been established, the committee will use the funds raised as the source for any reimbursements needed. All funds leftover will be considered profit.
As for whether or not you should be expected to work with these limits, that really is up to you, but I certainly would not. It's a worthy cause for sure, but the point here is how
business SHOULD be done (or in this case, not being done). I think a lot of people/parents forget that running a successful parent group is in essence a business (just think about how much $ you're dealing with on a yearly basis) and not just a social group.
Can you tell I'm venting a few frustrations of my own here?

Good luck!