Oxygen sensor

Let me start off by saying thank you to all that have replied when i post a question. You have been very helpful and i saved some money......Now, the oxygen sensor. I dont think mine is not functioning correct because i am burning alot of gas and i don't drive hard at all. Is the oxygen sensor costly and is it hard to install? Something i have notice is when i am on a hill the gas gauge decrease alot. Is that normal.1998 chevy malibu.

If you have it scanned on a graphing scanner the 02 activity can be observed and a determination can be made if it is working properly, also the fuel trims can tell a lot here too.