I have played the odd game a bit similar to Dino RPG where paying is optional but you get something for it (Travian was one example). But so far i never bothered to actually pay for them i just played for free.
I just thought for Dino RPG it's a lot better to spend the money early on in the game, but i wasn't sure that i liked it for like a month plus i am poor.
Although when you mention games like World of Warcraft i suppose i have spent a lot of money on online games. I played WoW quite seriously for 2 years (quit last year, i lead like 3-4 raids a week) before that i played a little known 2D MMORPG (called Darkages) for 5 years on and off these both had monthly subscription costs of course.