Installing central air costs a lot of money. Once it's installed, the
cost of running it is less than the cost of what you have now.
However, it would take more than 30 years to make up the cost. It
could even take more than 100 years. In other words, if it costs you
$1000 a year to run window units and $800 a year to run central air
that's a savings of $200. However, if central air costs $6,000 to
install, it will take 30 years times $200 a year for you to get your
money back.
So if you are comfortable now, don't change. If you are hot now,
consider the money.