Door Lock 97 Camry

I have been having problems turning my key in the driver's side door lock to unlock the door. Last year I fixed it by replacing the key (I figured it was warn). This year it happended again. I replaced the key and it seemed to work OK for a couple of weeks. Now I can't turn the key to open the door. It does turn enough to lock it however. I tried lubing the lock - didn't help. The key works in all other doors and the trunk. Anyone out there have the same problem? What was the fix and how much did it cost?