Bill, I'm going to assume you mean a fire assay, which is the standard for gold. 

There's assays for prospecting and then there's assays you can literally "take to the bank" for financing on a mine.  When there is a dispute between a miner and a refiner, there are "umpire assays" which will be done at labs like Johnson-Matthey.

Getting to your question about oz/ton and mining - that's partly an assay issue and more a sampling issue on the core samples. The core drill guys had better be doing their job well across the intended mine site to make sure they've pulled enough samples to get a good picture of what they're expecting. In Nevada, they pull literally thousands of samples in three dimensions to characterize the gold deposits. When they get to mining, the ore yield is pretty close to what they are expecting from the sampling and assay lab  results. A gold mine of any commercial size will usually have their own assay lab on-site because of the number of samples they pull, which will be dozens to hundreds per day. I'll ask my buddy what he reckons the in-house cost for a mine's assay is.

Ballpark figures for you to send off for an assay:

http://aallabs.com/cms/index.php?page=Full-Price-List

That's an independent lab in Elko. There are others. If you're ever seeking an umpire assay, I'd go directly to Johnson-Matthey out of Salt Lake. When you're seeking to end any discussion about how pure a gold sample is, you're best going to the Big Dog in the gold market.