[Yosei]

I thought of the SP investment as a rather bad choice due to the availability of life tap (mages solo'ing at higher levels just won't work well enough). You either duo with a cleric (tap/rest after the mob dies) or the tank take hits in AoE grinding, so health is generally not a concern associated with using LT. Aside from Burst/Fire other skills atm are rather useless any higher than l1, so the consumption are low as well (until you hit 49-70). P.s. so far I've been able to tap and kill nonstop duoing with a cleric. If the cleric is stingy simply throw a few silvers to cover for his/her stones (50 stones will last about 4-6 hours for my case) =)

At higher levels.. it'll benefit you. I don't use life tap, because if you use all higher levels its really only good for 2-3 extra skills. And a lot of clerics dislike life tap. Its like boy who cried wolf story.
When I grind, and I mean long grinds, without using Nova frost, I use about 2-4/81 stones. And with nova frost(with sp decrease) I use about 20/81. So I won't have to restone quite yet when I'm done grinding, so... all the money I made from drops from that grind just go to my savings =D
1 SP stone for me costs about 220c. that x 81 = about 17s.