Daily rates are $10/single and $18/double. If you pay in advance for multiple days there is a $2/day discount.
When doing a multi-day rental, you get a strip of rental passes, one per day. Each time you rent, you skip the pay line (which can be quite long at times) and just go right to the pick-up area with your rental pass. They will stamp an unused pass with today's date. If you return to that park OR go to another park on the same day, you again go to the pick up area and merely show them the rental pass with today's date stamped on it.
ANy unused rental passes are fully refundable if you have your original receipt.
Locations:
Magic Kingdom -- There are 2 archways under the railway tracks, take the one to the right, and while inside go to the left.
Epcot -- forward and to the left after entering (way before Spaceship Earth).
Disney's Hollywood Studios -- forward and to the right, looks like an old fashioned gas station.
Animal Kingdom -- forward and off to the right.