Funding for MUDs has always been an extremely difficult topic. You've got basically two options - either rely on financially stable retirees with way too much free time or rely on college students with way too much free time. The hosting cost is essentially nothing and setting up a dono button but accepting that you'll probably spend about 1-2 hundred dollars a year of your personal money (as the owner/operator) to maintain the server can cover that cost... but staff is where the real challenge is. MUDs live and die by having a good, fair, engaged staff of RPAs (role-play administrators) or whatever you call them... and those people need to absolutely love the game world because the expectation levied on them is either (with good staffing) 4-6 hours a day or (with thin staffing) 8-12 hours a day of labor to watch player requests, set up events - etc... and while mud player bases skew towards experienced players who appreciate and respect the work an RPA is doing there are always new players that can be abusive and even experienced players have bad days (especially if your game is perma death - then drama will occasionally dial way up). One other thing to keep in mind is that when you accept the mantel of RPA you're losing a lot of your ability to enjoy the game, you have access to privileged information and will either 1) metagame and be a terrible person 2) manage to drive your character in a direction away from plots you're personally managing (only possible with sufficient staffing) or 3) (the most likely) find yourself having difficulty playing because you know the metagame temptation is there and it's hard to compartmentalize information.

The TL;DR is that MUDs are freaking hard to staff for free - offering compensation (even if it's just an occasional 100$ gift card in thanks for a month of work) is a huge boon to the community... but given the size of the player base and the demands on staff actually paying a decent wage is pretty much impossible. In your 100 person MUD example the staff is probably somewhere around a dozen people - lets assume you have a staff ratio of 1:8 - that would mean that to afford 15/hr you'd need players (that tend to be logged on 4 hours a day) to pay two dollars an hour or sixty dollars a month. Employed people can shrug off this cost - but people with a full time job can't play MUDs... so you're looking at getting 60/mo out of college students and retirees and that's pretty impossible.