What are your vacation dates?
Usually you will see rate drops as you get closer to your departure date.
I'm assuming that you will need a return rental also? Have you considered keeping the rental for the whole week (or length of stay), sometimes you can find great rates on weekly rentals and most discount codes need a Saturday night stay so you can typically add an extra discount $20-$30 off a weekly rental.
This way you will not be restricted to just
Alamo/National on a round trip rental, all rental agencies could be searched for a great rate.
The added bonus here would be that you will have transportation if you want to do anything that Disney transportation doesn't allow for (grocery run, trip to the Outlet Mall, day at a non-Disney park - Universal, Seaworld etc, or maybe just a nice relaxing day at the beach)
FYI: we are flying into SFB on January 30/16 and have a 10 day rental booked for a Full-Size from Alamo using the Costco code and $20 Entertainment coupon for a total of $142.87 incl taxes. This originally started much higher, when I began looking in Sept 2015 we were looking just over $300 for this same rental. With 3 weeks to go, I still check prices daily incase the price drops again.
ADVICE: Book what you can afford now and check frequently, re-booking as prices drop (don't forget to cancel the old rezzie AFTER booking the new lower price).