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Hello, the absolute sure fire way to get a good price is to get 3 or 4 bids on the job.
Without seeing it personally and the range of issues involve, such as crane access, roof construction and all that I can tell you are rules of thumb for bidding projects (I am a retired HVAC contractor from California, Sacramento down to Modesto mostly. wotj 10 years in Los Angeles, and still consulting in the area to some degree).
If you can give me the model number of the unit being replaced I can look up the cut sheets for it hopefully and see how close it comes to matching the existing curb (you are calling it a 'build up')... that is the biggest expense in the replacement.
I will be out for an hour, then back here to respond.