We bought a new one from Besche last year. Same problem; cracked heat exchanger. I can't tell you exactly what the furnace alone cost us, because our A/C was on its last legs, too, so we decided to replace the entire system. Furnace, A/C condenser and the compressor outside the house. The whole system cost about 5,500.
We could have gone cheaper - could have had a new system for only 3,500 but we bought the best that they offered because I know that repairs are expensive, too. I wanted to buy a system that I could trust that wouldn't lead to a repair guy coming out twice a season.
If you are only doing the furnace, I imagine it will be expensive but it won't cost you your firstborn. On the plus side, the new system works WAY better than the old stuff, the house is comfortable, and we are burning half the oil each season than we were with the old furnace. Besche has always been reasonable with service calls, too.