Your least expensive option is to use one of the less expensive air mattresses (they usually aren't as thick as the more expensive ones either) &/or a sleeping bag under your sheet but on top of the regular mattress.
As long as you have the sheet over it, the air mattress will not get uncomfortable in summer. If you have a mattress pad over it as well, that's probably even better.
You may want to get some of the clamps to keep the sheet on the air mattress as I had trouble with mine slipping off on the twin size one I used on my cot when sleeping in the booth at Sherwood Forest Festival and Scarborough Faire this spring. Of course it may not be as much of a problem when it's on a bed instead of a cot.
Here's to comfy sleeping!