I think availibility is the issue. you easily find them for say ~470 online but nobody seems to have them in stock.
gm parts direct has them for $590 and that is the cheapest place I have actually bought from before.
just search the web for 9599037 that is one of the part nubmers. I am just not sure who actaully can get you one the fastest.
2013 ZL1 -sold
2013 Megacab 8" lift 38"mt's