I certainly use the thrift stores alot, being on a strict budget with 4 kiddos. I have found it gets expensive regardless though. I have gone from a 14/16 to an 8/10. I sometimes wear my old pants with a belt just so I can see that difference myself.
If you find a support group near you as well or people that live relatively close you can trade off clothes depending on the stage your in .. like someone who is out of a 14 can pass them on to someone moving into that size range. If you get 4 or 5 friends all doing that the clothes should make a full round before getting tossed.