I've played quite a few MMOs varying from the great (EQ, CoX) through the reasonably good (EQ2, WoW, LotRO, WAR) to the downright awful (VG), and tried a whole bunch of others that I forget.
The argument about the cost is simple - you get far better value for money than most games. Barring rare things like Civ, you might get, say, 20 hours gameplay out of a new game (actually, that's pretty high). I'll probably play an MMO for at least 10 hours a week, for at least several months, and the good ones I'll play for years (I've been playing CoX since the UK release). Yes, I'm paying more for the MMO, but I'm getting way more hours of gameplay per pound. Plus, decent MMO developers (eg Cryptic/Paragon) will provide a constant stream of free content.