In the < $50 range, I'd say either Radbot 1000 or Cateye 1100. Unless you have a Magicshine headlight, in which case absolutely the way to go is the $29 Magicshine taillight + y adaptor from GeoMan.
I actually went from a Dinotte 140R to the MagicShine. I think the brightness is about the same and the dispersal is actually better - it's visible farther to the side. The real win was using one battery pack.
__________________
Work: the 8 hours that separates bike rides.