Cleanings are fairly cheap but, as mentioned, fairly useless unless you have visible debris or some other major problem.
However, have your ducts sealed can save you a shit ton of money, although now we're talking about a much higher cost. I have a 600 c/f system, and I was losing just over 200 c/f due to the fact that energy was cheap when my house was built and sealing joints wasn't a priority. Now I only lose 30 c/f, and I am observing savings each month (I can't conclusively say how much until I observe a similar 6 month period this fall.)