I have a set of nice 40k mile take off z51 c6 19/18" wheels and tires from a 08 c6, tires are about half life and are good years I believe. Around what are they worth?

 

First off, the tires are probably not a good selling point due to the miles and probably old date code, provided they are indeed the originals. They just add weight and bulk for shipping. The new owner would most likely want new rubber. If they are newer replacement tires, and Goodyears, then that is different and would be an added value.
With new factory wheels going for under $2,000 (see below link), you will be looking at 50% off that (at the very least) even if truly mint with no curb rash or marks. There are quite a few sets on ebay in the $700 price range so that might be a good indicator of pricing. You can always try for $900 -$1,000 if your 'mint' description is 100% accurate. You never know. GLWS!!
http://www.ebay.com/itm/SET-OF-4-NEW...edf34e&vxp=mtr