You take more of a chance renting at the Disney parks (first come, first serve), but it still would probably be cheaper if you need to do a van rental with ramp for an
ECV. If you get there early each morning you should be all right getting an
ECV rental at Disney. The cost is around $48 a day. I've never had a problem renting an ECV at the parks if I come at opening time and I've done it that way for 12 years. You do have to deal with walking and getting to the parks from the parking lot. There are wheelchairs available for this purpose, if you need it. There are park shuttles, but you need to get to it from your parking spot. If you have handicap parking there's no shuttle; because, you are as close as you can get to the park entrance. You would also have to live with the fact you won't have an ECV anywhere else that you go to.
I've heard on these boards that there are taxis that can do
ECVs. I believe the driver would take it in and out of the trunk, but you would probably have to assemble it yourself and take it apart, as well; unless, you have a very kind taxi driver that will do it all. I'm not sure if this would be cheaper. Also, you wouldn't have a car available all the time; unless, you rented a regular car in addition to doing the taxi.
I really feel you should stay on site, then you wouldn't have the issue. You can still get an off site ECV rental at a reasonable cost that way. You would be able to use it everywhere you go. Even if you go elsewhere hopefully hubby could do the ECV a few times as oppose to all the time in and out of the trunk. Sometimes offsite is just not a good deal overall; you have to consider the whole picture. Even if it costs more onsite, it's so worth it to me.
Otherwise, you will have to bite the bullet on the cost of a van with a ramp. I'm not familiar with these kinds of rentals, but hopefully someone else here may lead you to a cheaper rental alternative that way. I really think the $650 price doesn't sound so bad. I would do a search and call every place out there to do a price comparison.