A $500 RC plane isn't going to be carrying any kind of load that can do any real damage.
Sure, you can pack an RC plane with some C-4 and just fly it kamikaze style into something, but it still couldn't be much more C-4 than the amount needed to blow the lock off a door. Explosives have weight, and RC planes can't carry much extra weight. Given the imprecision of flying one of these things any distance whatsoever, I would think you'd have to carry a tremendous amount of explosives to be able to reliably take out any sort of target.
I would think these would make for an extremely ineffective weapon. A truck bomb would be much more effective.
As for military drones, while it's fun to play "what-if", the reality is there's no practical way for anybody else to attack the US mainland with one of these. For one thing, they are extremely slow. For another thing, they are not stealthy. They would even show up on commercial radar. Heck, bottle rockets show up on commercial radar sometimes. And the FAA doesn't look kindly on unauthorized flights in commercial airspace.
That's not even mentioning the range. We fly drones in Afghanistan from Afghanistan. Where is somebody going to launch a drone attack on the United States from?
And lastly, drones are not difficult to shoot down. Lots of things are immune to heat-seeking missiles - that's why radar guided missiles exist. We've actually had several drones shot down ourselves, even in countries with zero radar coverage, zero opposing air force and zero air defense. These were shot down by guys looking up into the sky and getting off a lucky shot.
Look at it this way. How successful would you imagine a Tu-95 bomber run would be over the US mainland? I personally would expect that every one of them would be shot down - they'd be detected early, fighters would be scrambled, SAM sites alerted. Now replace the Tu-95 with a slower, less well armed drone with a lot less range. Why would you think it would actually be easier for a drone to get through?
I think we've got more important things to worry about, not least of which preparing for more conventional attacks from our enemies, or more "traditional" terrorist attacks.