Often the Comfort Inn Lake Buena Vista has deals for as low as $29 per night.
Check www.travelzoo.com for deals.
Any of the Palm Pkwy hotels should offer something in your price range.
Check on Mousesavers.com as well....I just got an email about a new deal at the Buena Vista Suites. The rate was around $85 per night including breakfast.
There is also a room only discount on the Disney resorts. The value level resorts will cost a bit more than $70 per night but you could then park your car and use the bus. That actually brings something else to mind.....
if you are staying off-site you will need to pay $12 per day to park at the Disney theme parks. So when factoring that in it might be possible to stay on-site.
Aside from that there are loads of other ways to save money. Having meals in your room instead of eating out....shortening your stay by a day or two.....eliminating park hoppers.