Level 1 of an ability requires 1 ability point, level 2 requires 2 points, etc. If you have a level of an ability from a talent, trait or equipment bonus, that does not affect the cost to learn the next level; for example, Jahan starts with the Scientist talent, which gives + 1 to blacksmithing and crafting; in that case, learning level 2 of either ability would cost 1 point, level 3 would be 2 points, etc.
Mater level skills require level 4 or 5 of the applicable skills ability. After character level 11 you start getting 3 ability points per level, so you'd have to save some points until the next level in order to invest 4 or 5 at a time.