OK, so for the cheapest rig in the world, just buy a recirculator pump without a timer. Buy a clip-on snap-disc aquastat and clip it onto your return line. Wire the aquastat in series with the pump. Obtain power from the light switch in the room nearest the end of the hot water line. Tada, enjoy. Not quite as precise or adjustable as a valve-style aquastat, but it will do the job.
563-2 - Taco 563-2 - Temperature Aquastat for "00" Series Circulators (95°F - 115°F )