Sorry, I was keeping things short as I was on my phone. Basically, I already know I could donate monthly to it so I had moved on to how much would I be willing to donate to it. That is where I ran in to a problem as, having no experience of this, I had no idea if a server is a few dollars a month or a few hundred dollars a month and that very much affected whether I would, or wouldn't, be willing to donate.
I can appreciate that it varies depending on the number of players we want (though hopefully this should reasonably scale) and the mods we want to run. However if you're looking at a ballpark-ish figure of a few dollars a month each then that doesn't sound too bad.
I have run servers for other games many years ago so know the problem of people promising to and not paying so I don't think it is unreasonable, at least initially, to ask for 3-6 months worth.