I am renting from Randy for the first week, then we take a four night cruise and then I need a ECV the last two days, what are disney's prices per day?
You might explain your travel plans to someone at Randy's and see what they might be able to offer you. It tends to be much more convenient to have an ECV full time rather than trying to rent one in the parks. This is especially important if you are planning on going between parks on the same day; often it is difficult to get one in the parks other than first thing in the morning.
Here's a link to the DIS page about touring with mobility disabilities. It includes info about renting ecvs at the parks.
Cheshire Figment had some really good advice though. Randy's Mobility might be willing to work out something for you and then you wouldn't have to worry about trying to get one at the parks.