But you are still paying a lot cheaper than me. (is due to my fault too because I was not aware that I can look for other company other than my original supplier, it is an expensive lesson for me)
I think most of these furnace/AC service companies are quite similar in price (600+). The only thing that matters most is their quality of service.
If you are a handyman, it's possible to simply buy parts (from them or from Ebay depending on how cold/hot the weather is) and install by yourself, which should save half of the cost.