Do you live in an area that sees humidity above 45-50% during the summer days? If not, consider switching to an evaporative cooler. MasterCool makes a really good one. Of course, your house has to be set up for it, so the initial install may not be cheap, but the operation and maintenance is far cheaper than refrigerated air.
I thought evaporative coolers worked with really low humidity, not higher than 25%?
Anyway, we had two ducting systems for A/C and wvap - the evap needs large ducts for a bigger volume of air.