no, there is no way to make relaying cheaper to implement, the fact is that it's pretty cheap really, like, my test relay, over about 3 weeks now it's accumulated a whole 20mb of data, which includes all posts that make reference to mine, and i have some, idk, primal says i have 300 followers... so all their kind 0s are in there too most likely, but idk, those are probably the biggest individual events you find on the network
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oh yeah, and that dates back 3 months of my activity too... so if you calculate on each user costing about 20mb/month of data you are looking at a pretty low cost to host 400 of them, i pay about 5 euros a month for a VPS that would handle that
i haven't worked out the payment system yet... currently working on a chat-bot based control system for admins and owners and i think it probably won't be much to add an interface in there to allow subscriptions, maybe a couple of weeks i'll have most of this implemented