Maybe $20/mo. The bandwidth is growing but not too bad so far, every once in a while I see a really huge transfer that makes me wince (seriously, who sustains a gigabit/s for five hours?), but I've still got some margin before I run up against the monthly transfer allowance. When that happens, I'll probably move the server to a provider that has a slower but flat-rate bandwidth plan, maybe Scaleway. Transfers would be capped by their network limit (100Mbit?), but effectively free.

I might also change the transit relay to throttle the connection somewhat after some total number of bytes have been transferred (but that takes actual code, and switching providers would be easier). I'm really glad folks are using it, but I do sort of think that >10GB at a time is a bit much. But I don't have a way to notify a given user about any sort of soft limit like that (the transit relay has no idea how much you're going to send ahead of time, and doesn't have a notification channel other than throwing an error).

I've also been meaning to figure out NAT hole punching for years now, and the thought has always been that hole-punching will save a lot of the server bandwidth by making direct connections happen a larger percentage of the time.