Wow, I did imagine pluto would be insanely expensive.
Another idea would be more the sponsorship side of things. Finding small farmers markets, community groups, permaculture events, small hackathons
... Etc. Stuff which is already aligned and somewhat present here, and sponsoring them, that way you could have 'sponsored by Nostr' at their events for quite a long period of time maybe. Or similarly, a beekeeper or farmer who sells produce. We buy their banner and in the corner it says 'find me on nostr' or something, so this would be a more long term and more targeted ad, possibly even way cheaper. These users are already here too.