Our TRANE furnace Model TUE080A948BD not working properly. Service person came in & said we need to replace the gas valve & gave an estimate of $900 ($250 for the gas valve + $650 conversion kit) + labor. He's saying that he can't find gas valve for this model so he needs conversion kit, which costs $650.
Would it really cost this much to replace gas valve?

http://www.aireng.com/clientuploads/Trane_Data_Guides/Furnace/tue.pdf


It is possible the valve is no longer available and thus the furnace needs a complete kit for the repair.

Now what I would do is contact another dealer and pose the same question OR get another estimate froma different company all the while never telling them of your problem. The price sounds high to me.