It depends on your traffic similar to a website.  More traffic = more cost.
Strfry can be run in docker, so its not that complex. It's only beneficial if you convince people to use it 

 IMHO, bigger trouble would be disk capacity than network limit (speed/capacity). VPS usualy don't have big disks.
Or do you mean to run it at home? Tor-based relay? 

 Yes to both disk and bandwidth. More users posting to your relay = more data