Scroll down, you will see a number of posts here on this subject. Also check AllEars and Mousesavers, they may give a breakdown.
To answer your question FWIW: Lots of variables-time of year, length of stay, type of room requirements, etc. Plus it depends on you and the type of trip you want. If you want all Disney all the time, need airport transfer and park transportation, stay on at a value. If you want a break from everything, plan on other things than WDW, maybe get a little more for your money (free internet, breakfast) off-property would work.
The car rental is a problem, as personally I would not stay off without a car. Never used hotel or community shuttles but my understanding is that they are very limited, just morning, late afternoon and evening, nothing real late if you planned on doing dinner or DTD. (Check National, Thrifty for rentals at 21+, though it may cost more. Car is just more convenient than airport transfer and shuttles. Even when on property, I still drive; faster/easier than bus to parks, other resorts.)
That said maybe look at the DTD hotels, Marriott Village or other LBV places. If the rate is enough of a savings vs. the value resort to off-set cost of the shuttle (some are free, some charge) it may work for you. The really cheap places are in Kissimmee (192) area; some may have free shuttles, but they tend to be older, lower quality. If you don't want to risk it, then look for the low season deals at a Value Resort.
