Quote: "Asking for a monthly fee is a serious liability anyway, given the server(s) which you have to keep running and maintain all the time."
Basically all MMORPGs have it in their terms that they are not liable for server downtime. As odd as that sounds, it's true. That's how MMO companies get away with launching a commercial product before it is stable, and there have been some launches even in recent history that were not really even playable when all of the launch player's free month was up. You will lose a lot of them, but you are not looking at a class-action lawsuit.
As far as the question raised, I am going with blomster: Have an item shop. This is becoming the massive norm for smaller MMOs on the internet. Why? Because it allows players to get sucked into your game
first, before you put your hand out to collect anything. Yes, you will have players that never buy anything, but a lot of them won't stick around. Good sellers seem to be temporary (usually between 1 hour - 2 days) boosted XP gain for a pretty hefty price. Also, gear and weapons usually sell well. Just try to keep the advantage reasonable, or the other players will complain and outcast those who buy the weapons (this is a VERY bad thing to happen in your game community, as your financial supports will feel
bad for keep the game alive). Even if there is a pretty big advantage to buying items, some games have survived.
Achaea (a MUD) has been running for over 10 years, and the real-money items you can purchase are insane and with no free equal. The way they pulled it off is that a player
can buy credits with in-game money, but it takes a very long time, and a lot of gold. The credits are put on a "Credit Market" by a player who has credits and wants gold, and bought by a player who has gold and wants credits. This is probably one of the better systems I have seen, because the players who do not buy credits have very little room to complain. If they work hard, they can have credits too without spending a cent. Some of the items in that game cost over $300 a piece, and people buy them
As for the low monthly fee, I would probably not pay it to play a small-ish indie game. I would, however, try one that is free with an item shop (and do on a regular basis).