My attic has 2 feet of cellulose insulation in it. On top are the air ducts and hvac units. It gets really hot up there (live in South Carolina). The crawl space under my house is always cool. I am thinking I could save some on cooling in the summer if I were to put the ducts in the crawlspace. What say you?
The first step is always to lower where the heat escapes into the house (for summer cooling season). You most likely have plenty of attic insulation. But you might check for other air leakage places (around doors, windows, pipes, electrical outlets, etc.) Less heat entering is less heat to cool. Shades or solar film over windows help too, as well as trees etc.
If the air ducts are insulated, then there might be some difference in moving the duct work to the crawl space, but most likely not enough to cover the cost to change it. If they are not insulated, then the cost of installation still might be higher than many years of savings.
Another possibility for savings might be an attic vent fan, which could lower the attic temperature some. Just lowering the attic temp by 10 or 20 degrees is worthwhile in many places. It means a little less heat getting through the insulation and the ducts would absorb a little less heat as well.
Along with the other answer, the cheapest solution would be to drop the overall temperature in the attic. With insulated duct work in an average temperature attic, the heat gain into those ducts is minimal. What you do get is when the A/C is first turned on is the initial hot air coming out of the registers. Back to the attic cooling, if you can install an attic fan with a thermostat will be the best solution based on cost effectiveness.