Have you tried Souper Salad on Berkeley St.? I've only been to the one at the Beth Israel Hospital, but when I worked in that area I used to go regularly for lunch and thought that it was decent.
209 Berkeley St, Boston, MA 02116
I think Sebastian's does pretty ok for the lunch crowd in the Financial District, etc. If you trend away from any added proteins, you can get an enormous salad, with as many additional veggies, nuts, etc. thrown for the base price (something like $7). Produce is usually pretty fresh - have rarely if ever had browned-edged leaves, etc. Cosi by comparison has only pre-set salads and they nickel and dime you for any additions.
Also near Fanueil Hall, Boston Kebob House has a pick-your-own salad of middle eastern treats and a salad bar that usually has a line out the door. I hate lines so I have never had the salads, but the middle eastern food is pretty yummy, weighed by the lb., and the price tops out at the 2 lb. price (I think like $11). If you choose wisely, you can make a nice meze plate for $7-8.