I was curious if there are any mud code bases out there which would allow customization of the code and also the option to either set monthly fee's for players or charge for some other services? From my understanding CircleMUD will not allow that, but are there some with less restrictive license? I could roll my own I suppose, but I was hoping to avoid reinventing the wheel so to speak if I did not have to.
If you're excited about a Java or python codebase, you may be in luck.
I once did some legwork to find something in pure C that is also full-featured, and came out dissatisfied. The closest thing is probably NakedMUD with every plugin you can find. And I wasn't thrilled with how it was structured. So now one of my side projects is a node.js CircleMUD-inspired codebase–but it's still at a very early stage.
I'd suggest using a codebase with something more permissive (BSD, MIT, Public Domain, etc), just in case.
The license applies only to the driver and not the mudlib so there'd be no requirement to distribute the game source to your players for example. I'm not sure what other objections there could be in this case?