It truly depends on what you want to be able to run. You have to figure out the load of the things you want to use. If you want to run everything in the house, you're going to need a pretty big generator and that's expensive. If you only want 100 amps, "whole house" LP/NG generators are pretty cheap - Generac's in the $3500-$4K range for a pretty nice unit. How ever, if you want over 200 amps, you're going to pay 4 to 5 times that much.
To start a 5-ton AC unit you need at least 15kW. That's about the last size that falls into the affordable range. 20kW and up start getting pretty pricey.
Both Generac's and Cummins exercise themselves weekly. The only thing you really have to be careful with is the batteries because they wear out after about four years, like clockwork. The upsides are big, though. They exercise themselves automatically, turn on automatically when the power goes out, and then turn off when the power comes back on.