Being as no one has replied to this yet I’ll suggest a few things.
Firstly work out if you actually are using all that resource now or if you could host your instance on a cheaper VPS for now and expand it to a larger one if/when it grows to actually need those resources.
Secondly I would suggest reading Darius Kazemi’s Run Your Own Social which includes a section on Funding. What I’ve generally seen is that instances will ask users to chip in $1/monthly or bimonthly. So essentially you crowd fund the server costs from the active users, those who make use of the service may feel this is a fine cost for them being as you’re chipping inthe work of running and securing the system for them.
I mean alternatively you could charge users for boosting toots or you could try and run google adverts or something on your instance - but there’s a good chance your users will just up sticks and move to an instance that doesn’t feature this sort of overt commercial interference.