Well just checking out the sale thread, going to my girl Cheryl's thread--she sells about 1/2 tsp samples for about 4. I think it'd be safe to say 1/2=$4-5. But if they are really hard to find rare ones the price would of course be a bit higher.
EDIT: Sorry I didn't mean to keep writing, other ppl posted before I hit "Submit". The ones above are good to go by I'd say.
Don't forget also that it depends if it is a matte, metal or non-matte pigment.
There is a lot less in a pigment jar of a matte or metal pigment, so the quantity is worth more.
So, limited and rare + texture of the pigment will make the value of 1/2 tsp vary and go up.
Minmium: $4 to a lot more for All-girl for example.
Hope this helps!