You probably wouldn't recoup 100% of the money. But so what? You'd have central air, which would make you a lot more comfortable, and would make the house a lot more sellable when you do move.
And besides, do the calculations. Let's say it costs $7,500 to put in central air--that's the midpoint of your figures. And let's say that you recoup 70% of the cost. That's $5,250. So your actual cost--the amount you'll spend that you won't recoup--would be $2,250. Over the course of 5 years, that's a cost of $450 a year. (OK, we're not counting the electrical expense.) Assume you'd be running the air conditioning for 4 months a year. That's 120 days. That's $3.75 a day. About the cost of a big fast-food sandwich or a coffee at Starbucks.
Maybe it's just me--and maybe it's just because the temperature outside where I am (Virginia) right now is 98 degrees, and is expected to hit maybe 103 tomorrow. But I've lived in the Midwest, too--in Missouri--and I know it can get pretty warm there, too. So, if it were my decision, based on the figures you presented, I'd definitely get the air conditioning.
And while I noted above that you're not likely to recoup all the costs, it'll definitely make your home easier to sell. And that's valuable, too.
Hope that helps.