.5 gram? Half a gram? An assay sample is bigger than that? To start off you need to figure out the cost of determining the value and apply it to each sample you have to process. For example if it is by fire assay, it will cost you more than a half gram of fine gold to run. Plus you will need at least .775 of sample to assay gold and silver by fire assay. What I am getting at is there is a minimum amount you have to charge just to cover the cost of analysis. Depending on the method that will vary.
If you are choosing expensive XRF, the cost per analysis is very very low but the instrument cost is high. If you are choosing fire assay, the cost per analysis is considerably higher in time and expendable supplies but the setup is less expensive. Finally if you are choosing to do acid scratch testing, if the material you are buying lends itself to that type of testing, your cost per analysis will be less than fire assay but time consumed will exceed the XRF.
Based on what you asked, I would be honing my scratch testing skills, especially for the very small lots.