Very informally (and now I've totally hijacked this thread ), we're talking somewhere around $105 a month to cover our hosting costs and get a real-time text chat server rolling. When/if we decide to start taking donations, I'll be laying all of this out in the announcement post.
EDIT: Since some are probably wondering what our exact hosting costs are each month, it's about $60, give or take a few bucks, for a t2.medium Linux, Route53, t2.small MySQL RDS (non-redundant), and transit charges.
[align=center][Throws money at again][/align]