On Skill point costs

You may recall that during the creation of my Fifth Cycle character, I had a bit of a go at the system for having different points costs for various skills, but that I allow GURPS to do the same thing with no comment.

In fact, many RPGs use differing points costs for skills, such as GURPS which bases the cost on difficulty, or BESM which uses genre utility to set the costs. The difference between those systems and Fifth Cycle is that they either have a set skill cost per level or a clear pattern to their skill costs.

For instance, take the sword skill. In GURPs this is a Physical/Average skill with a point cost as follows:

Skill level - Point cost
DX-2 - 0.5
DX-1 - 1
DX - 2
DX+1 - 4
DX+2 - 8

etc.

In BESM, combat skills are usually very useful, so have a flat cost of around 6 points per level.

In Fifth Cycle the sword skill is priced as follows:

Rank - Cost per rank
1 - 9
2 - 14
3 - 23
4 - 36
5 - 72
6 - 108
7 - 144
8 - 180

There's almost, but not quite a pattern.

9+(9/2 rounded up) = 14. 9+14 is 23. That's good. The next level must cost 23+14, or 37. No, it costs 36. Well, maybe the level after that costs 59? No, it costs 72. 2*36. But then, 72+36 does equal 108. 108+36 equals 144, and 108+72 equals 180.

Rank - Cost per rank
1 - x
2 - x+(x/2)=y
3 - y+x=z
4 - z+y-1=a
5 - 2a=b
6 - a+b=c
7 - c+a
8 - c+b

Comparing my set of rules to other skills show that they do match them, it seems the -1 for Rank 4 is due to rounding errors. However, there is no way that anyone can call that a logical way to work out skill costs.

So here's a challenge, dear readers, assuming we start with an even skill cost, say, 8 rather than 9, help me come up with a formulae that works for all skill levels and makes sense in a way that doesn't require Carol Vorderman to explain it.