Heater or Furnace repair cost can depend on a lot of factors and can vary widely, especially if your Heating system hasn't had regular maintenance during expluatation. Statistics tells us that the average price homeowners pay for a heater repair in the US, Nevada (Las Vegas / Henderson) is roughly $280. Some major problems can cost up to 1000$ and even more to fix them.
Eventually what you will pay depends on:
The warranty on your heater
The type of heater (brand, gas/electric, etc)
Service fees and labor costs
What replacement parts your heater needs
What kind of the problems your heater have