Race and profession will have a preference for certain skills which will be measured by a number, with this number representing a percentile (in the case of race -- profession is a different story). This is the percentage modifier these elements contribute to the total cost to raise a skill. For example, a troll may have a 160 for raising literacy meaning that trolls will contribute in increase of 160% to the cost of literacy increases.
Of course, that figure would need some further processing before it might have the honor of being called the final character preference for the literacy skill. Next we determine the preferences added into the equation by the primary and secondary professions. These preferences are slightly different than the race's in how they are represented. A profession-based preference is a smaller number representing the
change
that a level in this class would add to the total, with a little added as a base figure, for additional stability. For example, a barbarian has a literacy skill preference of 4 and a base of 36, meaning that a single level as a barbarian will contribute 36+4=40% additional SP to the cost of raising literacy, at only level one. If you were a level 10 Barbarian, however, this increases to a 77% increase in cost. This calculation is then performed for each profession. The primary profession's modifier is doubled (so it holds the truest influence), then all are averaged together (being careful to add 1 to the divisor to compensate the doubling).
This number now can be added to the race's preference. In this example, a Troll who has obtained the first level of Barbarian requires 200% SP to raise literacy. We're not quite finished, however, because now we need to adjust the figure by applying the player's level of practice to the cost. As the game progresses, obtained skills can be applied. Sometimes this is manually done. Other times this is handled passively. In either case the frequency with which skills find use contributes to a bonus reduction to the cost to raise a skill. The maximum level of influence this may hold is total cost * 50%. The bonus does not last forever, of course; if you begin to neglect these skills that bonus will slowly whittle to nothing.
It is important at this point to mention that the floor for final cost would be 50%, with the ceiling 200%. This would prevent outlandishly unbalancing figures from sprouting up.
So to figure all that out we take...
Racial preference [variable 'a']
Professional Preference [variable 'b']
Skill Practice Bonus [variable 'c']
and quite simply, (a+b)*c = total cost
Which is to say...
(85 + 25) * 0.92 = 108% base cost (a Dwarven Fighter training Literacy with a small amount of practice)
or...
(70 + 25) * 0.78 = 90% base cost (a Dark Elven Fighter training literacy with a medium amount of practice)
or...
(65 + 0) * 0.56 = 50% base cost (because 50% is the cost-floor) (Grey Elven Monk with a ton of practice)
or...
(160 + 40) * 1.0 = 200% base cost (Trollish Barbarian who has not practiced a bit)