Hmm the problem is that pas a certain number of players the bandwidth cost,elctricity and hardware maintenance cost might become a problem, I am not sure a one time fee could be enough to cover all these.
But if i understand correctly the server is sponsored by fragnetics, and is hosting not only planeshift but also FPS (as in CS, Q3 ...) servers, so they presumably have a respectable bandwidth available. How they get it though is another question

(btw I also prefer if it is free, and could go with a one time payment )