I think about the best self-contained sub-$50 taillight on the market right now is the Radbot 1000. It's twice as bright as the Superflash (1 watt versus 1/2 watt).

'Course, if you're gonna have a headlight too, and go for the Magicshine, Geoman sells the MagicShine taillight for $29, same price as the Radbot, and that fries anything short of a Dinotte (it's the same brightness as the Dinotte 140L).