You will be able to get your best deals by booking lodging in a vacation destination off season. Have you looked into the Jersey Shore? Look into Ocean City New Jersey. There are thousands of condo rentals super cheap off season (season is when kids are out of school in the summer).
It is only a 45 minute drive to Philly, 20 minutes to the wonderful Cape May and 10 minutes to Atlantic city.
Also look into rentals in Lancaster County (amish country). They are expensive in season but may be a bargain outside of summer. There is so much to do in the area (Hershey, PA, Longwood Gardens, Gettysburg, Valley Forge, etc).
I just used Airbnb for the first time and got a place during the busiest week of the year for $80/night.