What is the difference between 60,000BTU and 40,000BTU in a gas heater for a house, not a hot water heater. Is there a difference in performance and price?

20,000 BTU. BTU is heat input. 60,000 is 50% more then 40,000. You need to know the AFUE before saying one is better than the other . Of course there is a difference both in performance and price. More BTU, more $$. Why not have a professional HVAC person give you the particulars as there are many variables in determining what is needed.

Yep, agree with ma. There is an industry standard formula to figure this out. http://www.wikihow.com/Calculate-BTU-Per-Square-Foot