The multipliers are exactly that, a x8 takes eight times a sp for each level.

Training a level 1 from nothing to lv4 is 256,000. Level 5 takes an additional 256,000sp, for 512,000 total


Edited to add table...
to gain level/total
lv1 10,240/10,240
lv2 30,720/40,960
lv3 71,680/112,640
lv4 143,360/256,000
lv5 256,000/512,000

Higher multipliers(also called ranks), just multiply by multiplier.

I THINK that I've figured it out for the most part.
The (x2), (x4), (x8) stuff is simply the starting prices for the skill, but they all increase using the same formula.
L(x)=(starting price)(x+1), Where L(x) is the skill level you are trying to find the price for and x is the numerical value of the level
This seems kinda weird at first glance, but you are given all the info, so you just need to plug numbers in.

Let me give an example, using Grenadier.
Grenadier is (x4), so it starts at 40,960, so we know what the starting price is, now let's say I want to calculate how much L2 costs in skill points.
So I do L2=(40,960)(2+1)
You can easily simplify it to (40,960)(3), then plug into a calculator, and lookeethere, we get 122,880 SP, exactly what you said it was.
Let's do another example, just so I'm sure I'm not confuzle-ing you even more
Caldari Assault Dropsuit (x8) starts at 81,920, and I want to find the price of L3
So, L3=(81,920)(3+1) which equals 327,680 SP, which, if I look at my Skill Book, I find this to be correct.
Again, L(x)=(starting price)(x+1)

EDIT: I like to find the mathematical equation for these kinds of things, but the simpler method works just as well.

debacle, no spreadsheets yet