how much for o2 sensor?

so i got diagnostics on my galant and he says its an oxygen sensor and he said the other one is getting bad too so i mihgt as well replace them both, he says $280 for one and like $160 for the other!!! NO F***IN WAY!!!!

i was under the impression that they were like $100 each installed and everything is this guy just trying to rip me off or are they really that expensive?

my car is a 2001 galant gtz v6.

EDIT: oh yea ill post the error codes tomorrow i need to get them from the guy, which by the way charged me $70 just to plug the god damn car into a computer and get the error codes, probably took him 5 minutes tops, it pisses me off so bad... kinda makes me wanna be a mechanic so i can get away with this sort of stuff... its unbelievable.