I've been wondering recently how they come up with the points values for certain models. Is it arbitrary, or is there a system?
I think its a bit of a mixture of both, there is a 'standard' stat line with a standard points value, and adding or subtracting stats will increase the points by various amounts, some stats are given more weight than others. Then after that, any special rules are factored it. It is worth remembering though that every army functions differently as a whole, which is why you won't really find a perfect mathematical balance of points/stats across all the armies. What is good value in one army might be fairly worthless in another, so thats where the arbitrary point comes in. I assume GW also playtests the lists a fair bit before release and modifies it if something is too weak or powerful, though there often isn't strong evidence of tis in the final lists...
I believe that some things are always the same: a spear is one point, a wizard level is 35, but some others, like shields, change, and special rules are totally arbitrary.
Eh, knowng how GW have been working out lately they just sort of feel for a points value. Depending on how a model playtests they apply a points value for it. For skaven models they probably rate them differently than warriors of chaos models. They apply what special rules they have and how effective they are alone and in a unit. A skaven slave is not very effective alone, so their point cost would be less than a much more capable warrior. I think that is how they would do it.