Need more info.

Is yours oil, gas, propane or electric?

Does that $600 include labor for the heat exchanger swap? I've swapped them and I'd rather swap the whole furnace than a heat exchanger.....

One thing's for certain, if your exchanger is bad, get it fixed pronto. You'll be breathing CO pretty soon if you don't. And you won't be breathing it for long......

....I hope you have CO monitors in your home.