If you are staying @ Hotel Alma Domus in Siena, Rick's recommendation of Osterria la Chicchera is right on for good cheap food & 1/2 litre of wine for only 3.50 euro. Keeps the usual Italian hours of operation. Sometimes, I wanted to eat earlier that what most restaurants opened after a hard day of touring. Right across the street from the above restaurant is Zest. They are open all day, from about 11:00am on. I had a wonderful large piece of veggie quiche, salad, bread with oil & balsamic and bottled water for under 10 euro. Owner is very nice and speaks limited english. www.zestbarsiena.com
Ate at Antica Osteria Da Divo last week, it was awesome
Taverna La Tallina is good also-not real cheap though. I think it was a 5 course meal with wonderful Tirimisu for dessert. Limoncello was complimentary. The restaurant is tiny and reservations are needed.
There are also a bunch of fun places around the rim of il campo if you want to sit and people watch