Does anybody produces a reasonably priced fan controller with sensors
for outdoor temps that is smart enough to run the fan only when it makes
sense to do so (e.g. when the outdoor temp is more than a specified
number of degrees cooler than the attic temp)? Or has anybody built
something like that for which they'd be willing to share their design?


Sure, what you want is a differential controller. They are commonly used in
greenhouses, so search for "greenhouse temperature controller". Here are some,
about halfway down, look for the Dayton controllers:

http://www.littlegreenhouse.com/acce...controls.shtml

--
Dennis