Corner of Dartmouth and Boylston
Also if you go the other way, you will hit Bar Lola (corner of Dartmouth and Comm Ave), which has decent tapas and good drinks. Can be affordable if you play it right. Don't get sucked into Brownstone... Coda is the best on that corner of Dartmouth and Boylston. Also if you walk a few more blocks towards Hynes, you could try Cafe Jaffa (btw Boylston and Newbury, on Hereford I think?), which serves large, pretty inexpensive plates of Middle Eastern food. It can be very tasty! But seriously- split these dishes and eat family style, and you will get a lot of food for a great price.
Not much in that area qualifies as good or cheap. Coda is a good choice.
Heading to the South End is your best bet. In addition to Coda, Anchovies does relaible Italian basics at local bar prices. It might be a bit far for your location, however.
In Back Bay, Laurel does pretty good food at prices that are very reasonable for the area. Not special but a good compromise.
If you are arriving early or late, the bar at McCormick & Schmick has a special menu for "snacks" that turn into meals. (A burger for $1.95.) I believe that it runs from 4-6PM and after 9PM