It's true that, on the long run, paying a monthly fixed fee results cheaper than wasting tons of cash on item mall stuff. Specially if the cash shop stuff creates an imbalance in the game, be it faster leveling or unbalancing powers on PvP, some people can get to burn some insane ammounts of cash (the guild leader of a guild I was in on Holic got to waste over 100 usd on the cash shop, nuff said...).
But not the items persay, the imbalance gets bigger if the item mall stuff or the item mall currency itself are tradeable ingame, as it makes players that are able to buy it whealthier in the game, since selling the item mall credits can ammass them a lot of ingame cash fast (that's the case of Talisman Online and Ogre Island for example).
I have to say that the only cash shop I have ever made purchases from was not on a MMORPG, Dragonfable. 15 bucks for the full content and 10 two years later for some dragon coins... and I'm happy with it.