Dennis couldn't have said it better. It's truely hard to say (not that I could give you one even if I came to your house ...)
Anyways, what I want to add is that the best thing to do is get 3 estimates. As each Estimator comes to your house, you'll start to ask questions. Those questions might be like the following:
How come Estimator A is charging me $ABC and you're charging me $XYZ? How come you want to install a unit from MFGR. A and Estimator C wants to install a unit from MFGR. X? Why do you want to install new piping but Estimator A wants to re-use the 'old ones'? How come your sq. ft. 'number' is 20% bigger than Estimator B's number? And so on ...
The 'idea' here is to 'educate' yourself by listening to the Estimators and start asking questions as you hear more from the others. By the time you hear from your 3rd or 4th Estimator, you'll have learned an awful lot! Just make sure you can get an estimate on-site. It's best in case you have questions. Tell that to the Estimator. Remember, it's an ESTIMATE you'd like in the least; not a 'quote'. (If you get a quote on-site, that's best! But if you can't, you can always get 'some figure' from the Estimator and then have him call you back with 'the details.)
Others may have more to offer. My best to ya and hope this helps.