Prefix "T" ran from 1974 to 1978. I'd guess 1975 on this one. They were good guns, but there have been a lot of them made, and value is dependent on the condition and the buyer's desire for the gun. There was a 3" Magnum model available form the 1950s on.
I'd look at comparable guns on GunBroker. Figure face to face is worth $50 to $75 because there is no postage, FFL fee, or doubt as to condition. Just for a point of reference I got a cherry 1976 Magnum version for $245 delivered 19 months ago.
Oh, by the way, with that 'M' on the end of the serial number, that gun left the factory as a Magnum model, and it should say so on it, but you would need a 3" barrel to take advantage of that fact. The receivers were the same anyway, just the ejector and ejector spring were supposed to be different.