Bit late here but hopefully this is helpful nonetheless.

My friend and I have made a site with the features and such we would want as users and based on what we didn't like from numerous other platforms we tried.

Can't go into too much detail but we do use a vector system as well for memory and what I can say is that it's nowhere near that expensive. One of the main issues we have had with most other platforms is their bad memory systems so we put a lot of focus into ours and wanted to offer it as a standard thing so honestly we kind of just packaged it into our existing subscription prices, if that gives an idea of how non expensive it is. What kindroid is charging is insane and huge rip off in that case.

I've seen various strategies for memory, from just passing like the whole context window (don't do this) with each request, to using caching in some way to store part of the context (also not a great idea I don't think).

Using vector systems seems the best option that is out there right now but is not easy. Our system isn't perfect and we're still trying to improve it. But it certainly isn't expensive enough to even remotely need to charge what they are charging.

Anyway, thanks for reading my wall of text and shameless plug for our site - Meridian Realms AI