Just got quoted $5000 to service my HVAC system. Is this reasonable?
These questions are notoriously hard to answer because prices vary so much depending on location and house, but there is one answer that is correct for all of them:

Unless it's a small job, and you know how much it should cost, you need to get multiple estimates.  Wether it's the water pump on your car or an air conditioning compressor, get two or three estimates.  Anyone that doesn't want you to get another estimate or tries to pressure you into a sale is trying to cheat you.  It is standard practice to get multiple estimates on jobs that are this expensive, and the contractor should expect it.

Some repair people are good at inflating prices so that the cost of the repair comes within a certain range of just buying a new system.  They know what percentage they need to be at to make it seem like just getting a new one is the "smart" choice.  Multiple estimates are important because then they can compete on the actual repairs rather than scaring you into a bigger purchase.  Maybe your system is toast, and maybe something about the install makes it extra hard and expensive, but again, multiple estimates will show that as well.