We (a friend and I) just got back from a VERY budget-friendly trip to Williamsburg.
We drove down from NY (10 1/2 hrs on the way down and 11 on the way back!) on a Friday and ate at the Whaling Company, about 5 mins away from Busch Gardens. What an amazing meal! Fabulous crab cakes and white fish. The portions are huge, prices are fair, and the service is quick and very friendly.
We roomed at the Quality Inn, which we had our reservations about, but we found it to be clean and perfectly fine for two nights. We had free wi-fi for only $58 a night. The room was a bit cramped, so people who appreciate their space may not love it, but like I said, for two nights, it was fine.
We went to Busch Gardens all day Saturday (skipped the water park) and had a fabulous time! We put our bags in a locker and rode the rides for a couple of hours in the morning, braked for lunch, retrieved our bags and walked around exploring, looking in the stores, and taking pictures for the afternoon. That night we went to Colonial Williamsburg and took a ghost tour. The tour lasted about an hour and was fun but not overly scary. Seeing the buildings was a lot of fun at night!
The next morning we woke up early and just walked around Colonial Williamsburg (for free!). We may have snuck into areas we weren't supposed to

but I had to take pictures of lambs! Considering we paid zero, we got to see much of the plantation and I'm not sure if shelling out tons of cash for special tours is worth it. I was curious about the inside of the buildings, though... We slipped into stores and bought fun souvineers for friends. Around 12, we left for the long drive home
Overall, we spent:
Gas: $160 (We filled up about 2 1/2 times)
Hotel: $130 ($58 a night plus tax)
Road trip food: $40 (way down and back)
Whaling Company dinner: $50 (total for two people!)
Busch Gardens tickets: $120 (for two)
Ghost Tour: $24 (for two)
Late night snack at Hooters: $20 (so much fun, even for two girls!)
Souveneers: $60
Split between two, not bad at all. A trip to Williamsburg on the cheap is definitely possible. As you can see, gas was our most costly expense.