[Mathulhu]

It costs 18 points to raise one Path skill one level. This assumes you have spent 4 points in each of Thaumatology and the Path skill.
Those 18 points get you one rank in each of Thaumatology and the Path skill and one level of Magery.

If you want to increase all of you Path skills it's cheaper to buy IQ and Magery for 30 points.

My counter question would be how much should it cost? Given the huge range of effects that can be achieved with even a single Path it seems appropriately costed. In my experience RPM mages are amazingly effective, sometimes even disruptively so.

There is also equipment and Perks that are spell specific that can be bought for an even more focused spell caster.