How much should it cost to replace transmission? on 2000 Chevrolet S10

Mechanic told me I have to replace transmission for $2500-$3000 to pass inspection...This code has been on for a solid year if not more and the only thing I notice is a very slight up/down sound during the 55-60mph range. I don't notice anything with the performance or gas consumption. It's not janking or giving me any problems whatsoever. Code said the transmission was slipping. How much should a new/used transmission cost and is this guy trying to scam me? He's already talking bout me getting a new car or paying the a huge bill. Only 110K miles. Right now I'm in between inspection. He cleared the code and told me to drive it for a day to see if the service engine light will come back. Also told me oxy sensor needs to be replace...Could it be as simple as just relacing the sensor?

Asked by for the 2000 Chevrolet S10
Take our S-10 to a real repair shop and have a real MECHANIC with a real scanner check it out! I know nothing about these emission test that have to be done as I live in VA. where we do not have tham but that sounds like total BS!!
I think it's a P1870 or so code; It's mostly the valve body issue. If it's got metal in it, then it would be better to rebuild it. Around $1000.00, Jasper is $2000.00.
Kinda hard to say without a scan test and code numbers.