Toyota Repair: 1998 Avalon XLS O2 Sensors........., o2 sensors, avalon xls

Question
I have read on here that a lot of Toyota's have problems with O2 sensors and sure enough my check engine went off about a week ago.  Went up to get it checked out and it came back bad 02 sensors.  I am a poor college student without a lot of money to spend and from what I've read on here they are about $300 to replace (is that true?).  Autozone said it effects gas mileage a little bit but they didn't seem to concerned about it, should I be? Thanks for your time.

Answer
Yes it can affect gas mileage, this is an emission control problem and can also affect driveability to some degree but the car is driveable without causing any major concerns.
The biggest problem is that the car won't pass any kind of emission test in this condition. The price seems about normal for one sensor replacement.