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?

1 answer 2 comments
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.