Define pricey...? $100 for two with wine? Less than that?
I love Clio, but it will run about $150.
My favorite restaurant in the city is Hamersley's Bistro which is technically in the South End but not more than a 10 min cab ride from most Back Bay locations. You can eat there for about $100 if you either don't drink or don't do multiple courses. For my money, it's the best food, service and dining room in town.
Tapeo on Newbury has amazing tapas if you like that. I love the Spanish atmosphere.
Via Matta is also a delight.
Brasserie Jo is another good one. Haven't been there myself but know lots of people who have.
If you want to venture away from Back Bay, I'd go to Sel de la Terre on the waterfront (near the aquarium). You won't break the bank there, and it's like being transported to Paris.