[josh_f]

The other possibility would be to have the cost per level decrease as the level increases. If it were cheaper to go from 4 to 10 then from 1 to 4, people would be more likely to specialize. I'd probable suggest making the total cost to get to 10 a 120 points with two-thirds cost being the levels from 1-4.
Another formula that might work would be a flat rate per level equal to the starting path cost with a minimum cost of 10. In other words an archmage would pay 10 points for every level; a molch would pay 10 points for every fire level, but 80 points for every non-fire level. Like my earlier suggestion this boosts the mages and makes it more cost effective to increase a pretender starting path than to add four levels of air. (unless you have a mage type pretender).