Good question!
Mine's a small instance and runs on my existing infrastructure, so my only real cost (aside from a crazy amount of unpaid time and stress) is the domain name which is about $20/year.
If I moved it to dedicated infrastructure, I've estimated it would cost me about $65/mo for just the backend, UI, and database services (to maintain the same level of performance, anyway. Could probably host it for less and take a performance hit). Object storage for pict-rs would probably be around $10/mo since I force it to use webp
and have a 512 KB limit for user uploads.
Those numbers may be a little high, but they're based on my existing VPS provider which has amazing SLAs and uptime.