ORIGINAL: rainedav
That's really hard to answer. There's probably about three or maybe four members at every club that still build. The AMA web site says there are over 2500 chartered clubs, with over 170,000 members. I'll guess between 7,500 and 10,000 AMA members in the US who build from kits and/or plans.
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For every AMA member, there are probably 20 to 30 folks that build and fly models that have never belonged to the AMA, nor will they ever. This is just a WAG, but it has been my experience over the years. Some folks are not "joiners". Nor will they ever post to the internet, nor send a letter to an R/C magazine.
When the ARF/RTF craze is over, the builders will remain behind.
We tend to think in the terms of our own surroundings. We in clubs and on the net tend to be a gregarious bunch, but we are greatly outnumbered by folks that participate actively in the hobby that have no official connection to our organizations, nor our clubs. I wouldn't underestimate the economic power that these folks unknowingly wield.