Stars Without Number
The following subsystem allows for Skill Points to be spent on improving Attributes. First, a note about skills in the base system:
With the Improved Expert, the base skill set is 2 points per level for Warriors and Psychics, 4 points per level for Experts. (The Small Group bonus adds +1 skill point to each). Since Experts should be no better at improving their Attributes than any other class, the skill point costs for Experts are doubled.
Attributes are raised one step at a time. Raising an Attribute has a cost depending on the new value of the Attribute.
New Value Cost
4 6 Skill Points
5-8 3 Skill Points
9-13 1 Skill Point
14-17 3 Skill Points
18 6 Skill Points
(The underlying idea: Removing penalties or gaining bonuses is very very expensive, but moving around in the 8-13 range is quite cheap.)