Five bucks sounds about right to me, though that might be too much currently considering most one-time admission-fee relays (to my knowledge) charge less than that. Perhaps a slightly lower monthly amount, say 2$-3$, with some value-for-value system setup, not sure exactly how to pull that off though.

You would have to prove you have a great relay and establish yourself first for people to want to freely donate for example, and survive the competition in the meantime.