I have no idea how much he has or has made but I'd have to think it is more than most people at his age. Prudently invested he could be just fine long term.
Furthermore, in today's world of hyper-marketing and consumerism, there is no way someone like Galen with his name-recognition can't carve out a lucrative post-running career in a niche-market (race director, university coach/director, speaker, product specialist, sport ambassador). Potentially some kind of fun, piecemeal career that emphasizes his records/finishes while he develops/displays his personality outside of the harsh light of training, competing and trolls.
Given his running history and close association with Nike/UO, I would be surprised if he can't find a light/minimal-effort, six-figure, "visibility" position at either institution. Heck Coe did gyms, Symmonds is doing tanning salons, I'd bet Rupp has some capital to start something up.