I bought Scale-Color from Scale75 paints, They work for both airbrush and brush. Although I haven't yet tried them with the airbrush, I have heard that this is a hard range of paints to learn airbrushing with because they are very thick and need a lot of acrilic thiner.
However I bought them because they have really bibrant collors, they dry very mate, and their metalics are simply the best out there. I have used them with a brush ( to paint my golden serpent entry ) and I can't recomend them enough! They are simply the best paints I have used this far ( keep in mind that I dont mind having to thin them )
If I remember correcly the price for the Scale75 pot is about 3.5USD which is a little beter then the GW air range at 4.25USD.
I have also heard thet the Vallejo Air color is a good alternative, however I have never used it in any way so I am not sure about how it works.