It's $60/day. You can reserve ahead of time.
We just came back from the cabins. We rented it. For my family, it was worth it. I have two small kids and we're constantly running slow. The ability to jet over to the boat to Magic Kingdom or the main transportation center to catch a bus to the other parks is worth it. For us, the added wait times of the internal bus system and impatience of small kids is a factor.
That said, if your party is older (and timely in the morning!) and doesn't have any issues with the bus or waiting perhaps a bit more, then you can probably skip the expense.
If you decide to rent a cart, factor that into your hotel costs when you compare the cabins to other resorts on property, particularly if those resorts have a monorail or boat system (or, like those on the boardwalk, a simple walk to Epcot).