How long does it take to make money back on a new air conditioner?
It'll depend on the SEER of the current model vs. the one you're changing to, how much you run your A/C, and the total cost of the installation.

When was the last time the current unit was inspected?  It's possible that it might have lost pressure, which will affect its efficiency, but you typically notice that the system's not able to keep up with its cooling.  Of course, depending on the coolant used, it might not be that easy to get a recharge, as the regulations on freon have changed over the years.

(note -- my knowledge of A/C recharging is based on dealing with cars, I've never done a whole house, as I have window rattlers, not a centralized system)

What I'd personally do is try to identify the model &amp; SEER of the current unit, so you at least have an idea of how much of an improvement you'd get.  I'd also contact your local utility company, as most either have a program to check for leaks &amp; insulation problems, or can tell you what companies in your area can provide the service.

Sealing drafts is likely going to give you the quickest ROI as it's a DIY type thing, and it's going to be useful if you change out the unit or not.