The current system allows small gains between sessions providing progress an advancement. If you were able to increase characteristics with a huge cost people would be all over the boards whining about why it costs so much.
Plus, who wants to play 8-10 sessions to save up for a +1 to a characteristic. It has been proven time and time again in tabletop and video games small progression advancement is much better and preferred by players at large.
That being said it's your game and if you want to have the ability to buy +1 characteristic I would do a flat fee of 100 or 150 experience. If you do a cost multiplier most likely all you get is everyone buying up their low characteristics and everyone will be clones of each other. Having players with a 1 & 2 characteristics is important to player diversification. If you make it easier to raise the 1 & 2's you will lose that.
Edited by archon007