As noted, there are two considerations. The first is whether you already have ductwork that can be used (for instance, if you already have a forced hot air system), or whether you need to install new ductwork. The latter, obviously, raises the cost substantially. And you may also be faced with a variety of choices about where the ductwork can go--more expensive to keep it hidden, for instance--a concern in older and historic homes.
The other issue is the unit itself--the brand, and the tonnage...and the installation cost. Get a decent unit intended for consumers, not one for builders. The builder's quality units are cheaper, but often don't hold up as well.
Finally, get multiple bids. It's not just that some HVAC companies are cheaper or more expensive than others. It's also that labor plays a large role, and if a company is experiencing a slow period you may get a very good deal just so the company can keep its crews busy.
Hope that helps.