Total price for every skill maxed (from level 1 (except dungeoneering)) on Gold4RS = $9399.30
Total price for dungeoneering levels 1 - 99 on Gold4RS = $18450.09
See a problem here?
I'm not too big of a fan of it. Like others said, it has potential for a good mini-game, but leave it at that.
If they add more dungeons aside from Daemonheim then the skill may get more interesting.
The rewards are nice though.