Actually, with all of the insurance coverage plus a tank of gas, that might not be all that bad of a rate. In my mind, I was thinking $70/day for an SUV with no drop charges, but that would also be with no coverage and no gas. So your deal might be much better, depending on what it'd otherwise cost to get insurance coverage for the U.S. (Not sure what your credit card or personal auto insurance would cover.)

There are three ways the agencies price one-ways: (1) very high daily rates (I'm talking $100+ for an SUV), with no drop charges, (2) "regular" daily rates, with a drop charge, (3) low rental rate, but high mileage charges. Given your length and distance of rental, you actually want the drop charge pricing. So the $300 in and of itself isn't necessarily a shock - just consider it as part of the overall pricing.

The other option you have is checking rates as a 5-day EWR rental + a 4-day EWR-MCO rental. You might save a few bucks that way, and then you can consider the possible savings vs. the inconvenience of having to swap cars.