Completely depends on model and configuration. A wood stocked T3X hunter in .25-06 will bring more money than a Sythetic stocked T3 in .30-06.
I generally pay between $500 and $750 for the ones I buy used and that is completely dependent on caliber, configuration and of course condition.
I personally would pay more than that unless its a Varmint or CTR model which always command more.
You can regularly find them new for about $700-$750 for a stainless one. I overpaid for one more than I normally would about a year ago because I wanted a T3X Stainless in 6.5 and someone on here had one listed for a while.
It is funny cause you look on gunbroker and a regular old blued T3 that is older model will be listed at $800+ like they think its a rarity. Truth be told, i think when shops buy those and Sako's, they dont really know what model they are looking at so they guess and pick one out of the Gun Values book. Got a steal on a model 75 Classic at a local store cause they though it was a AV. Same thing with a 75 Hunter on Gunbroker.