Check sites like Expedia.ca, Hotwire.com, and Travelocity.ca often, as rates will fluctuate. Priceline.com lets you bid on a price, but I've not gotten any reasonable rate from them (their last recommendation was that I raise my bid to $115 per day - for a small car- ha!) My wife and daughter are flying out tomorrow for a brief 4 day visit. I made the mistake of buying them plane tickets as a Christmas gift, before I scoped up car rental prices considering the time of year. They are flying into Sanford airport, and because it's a Sunday, and they arrive late, the rental fees are absolutely ridiculous. Most sites, including the aforementioned, want between $600 and $750 for a 5 day rental (small to mid size car). I originally booked with Costco at $415 (still high, I rented a car for 5 days in October through Costco out of MCO for $115 total), but after getting some input via this forum, I booked with Budget, using a coupon code, for $268. Still high, but half of what they are asking.
Once you find your lowest rate over the next couple of weeks, I would book it (as long as there is no cancellation fee - i.e, Expedia) then keep looking. We will be driving down in March, but I imagine the rental fees will climb slowly. Good luck!