I recently had to have a fan capacitor replaced for about $400 including emergency off hours surcharges -- my wife wanted the problem fixed instantly because she didn't want to sit in a house for 2 days with no heat while I figured out how to do things the cheap way. I must admit it was nice having a guy over within an hour of the time we noticed no heat and have the whole thing fixed within another hour.
I know that people normally use an economic rationale to replace things, and that repairing is usually more efficient. However, the experience got me thinking. The unit is 18 or 19 years old, so I was wondering what I should do if a more expensive component like the compressor or reversing valve fails?