Did the mechanic verify that the sensor is bad? The computer does lie occasionally. Perhaps get a second opinion.

There is no escape for a bad sensor. It tells the computer how much gas to inject in the engine. I'd argue that it's the single most important sensor in the car to have working properly. Universal sensors are cheap (under $50), and a direct fit should only be a smidge over $100. Get the part, bring it to an inexpensive shop, shouldn't cost you more than $150 I'd reckon.

A bad O2 sensor will cost you more in gas, possibly wreck the cat converter ($$$), and could strand you. The math is pretty easy. I'd be sure that the sensor is bad, first. Cinchy test with a voltmeter that a smart mechanic would do.

Good luck!
-Ryan
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Athens, Ohio
1987 245 DL 324k, Dog-mobile, E-codes
1990 245 DL 137k M47, E-codes, GT Sway Bars, GT Braces, Dracos
1991 745 GL 304k, Regina, 23/21mm Turbo Sway Bars
Buckeye Volvo Club