I've spent the last 2 days successfully installing central air conditioning in my home, after 21 years with a swamp cooler. By doing it myself, my only costs were about $1500 in components and needed hardware, plus a few hundred bucks to a friend with an HVAC license to help install it and buy the parts for a lot less than retail. By my calculations, I saved at least $2000 over awarding the job to some AC company. While at it, I installed a digital, computer-controlled thermostat, as well as performing a thorough furnace/blower cleaning. I was willing to live with the swamp cooler for a few more years, if I could have diverted the money toward astronomy, but incredibly, my wife thought that air conditioning was a much better investment. Go figure!