This posts is a follow on from what I said earlier about language
/ music / art / acting skills  skills the character has learnt how to do, and
can perform to a consistent standard. This was occasioned by a character wanting
to learn the lute, and I saw no sense in calculating a bonus to performance
rolls. I wrote:
Character performs some skill they have learnt how to do, and can perform to a consistent standard. For example, language skills. People are usually “basic” (know a few words, basic concepts but lots of misunderstandings and lots of blank looks), “intermediate” (with effort can generally make oneself understood, can understand someone who is cooperative, lots of halting and repeating), “conversational” (can have conversations with people) or “fluent” (conversations are flowing, but accent and odd usage clear), or finally “native” (indistinguishable from native speaker). It makes little sense in an RPG world for someone to have expertise in excess of that. I’ll take this as a model for a wide swathe of skills: languages, music, acting. In these you generally do not roll dice – your skill level determines what you can do – and I won’t bother with rules for dice rolls until an issue demanding one comes up in play. Hence attributes (aptitude) should contribute mostly not to improve your chances of performing a task, but instead to make learning quicker.
What I'm looking for is how to make "attributes
make learning quicker" sit naturally alongside the more standard skills
(e.g. melee skills) where they improve your dice roll. To get a good fit, it
is important to understand what +1 attribute bonus gives you in the standard
system. How much is a +1 bonus worth?
The standard skills in Explore work as follows:
The skill is resolved by rolling 2d10 (open), plus your skill rank, plus the relevant attribute.Attributes have a bonus of between +3 and 3.
The total cost for skills at rank 1,2,3,4 etc. is 1,2,4,8 etc. (i.e. the cost for new ranks is 1,1,2,4 etc.).
10XP gives you 1 skill point to spend.
Note that in this dice system for every +3 the
chance of failure is halved.
Initially it would seem that since +1 attribute is
the same as +1 rank, and +1 rank costs twice as much, so +1 attribute should
halve the cost.
However in combat several skills combine to improve
your performance, by doubling the number of skill points spent you can afford
to increase every skill by one level, which means you get a total of +3 distributed
around the various rolls in combats (e.g. +1 to hit, +1 to parry) which in play combine
to be as effective as if you got +3 on any individual one. From this perspective
it would seem that +3 attribute should halve the cost.
Hence... we split the difference and say that +2 on an
attribute will halve the cost.
So now the solution for these skills is a cost multiplier, which halves every +2:
The cost multiplier is made easy to apply in practice by grouping the skills under the appropriate attribute (memory or intuition) with the appropriate multiplier marked in for the whole section. You sum up the cost under each section and then apply the multiplier.
Attribute Bonus

Cost Multiplier

+3

x2

+2

x3

+1

x4

0

x6

1

x8

2

x12

3

x16

The cost multiplier is made easy to apply in practice by grouping the skills under the appropriate attribute (memory or intuition) with the appropriate multiplier marked in for the whole section. You sum up the cost under each section and then apply the multiplier.
Note that standard skills already have a cost
multiplier of 10, this is just a rephrasing of "10XP gives you 1 skill
point to spend".
For an example of calculating costs, consider someone with 1 AG, +1 IN with skills Melee (AG) 3, Parry (REF) 2, Athletics (ST) 1, Lute (IN) 5 . Agility 1 gives them 1 on Melee, whereas Intuition +1 reduces the cost multiplier for artistic skills from 6 to 4. They've spent 4+2+1 = 7 points on standard skills, 16 points on artistic (?) ones, with cost multipliers of 10 and 4, for a total of 7*10 + 16*4 = 134 XP.
I have made these skills cheaper than the standard
ones because most skills are very broad – athletics, acrobatics etc –
whereas these skills are narrow. They are specific languages or specific
musical instruments. Hence the cost *should* be lower.
In these
skills there is a max level of 5 (as per the introductory quote above), so if you have +3 in the relevant ability you
can be maxed out for 80XP  so you could have this when you start if you wished
(as you start with 100XP). At the other end of the cost scale, maxing out in a spell
list (10th level) is 10240XP, which is the cost of 128 different languages!
Importantly, this whole aspect can be freely ignored until the
point where you have a player who wants to learn the lute!