In smelting concentrates a very common way is to sell them to the smelter, thus realizing the value at that point.  As in selling anything some expenses are incurred in locating a smelter and negotiating a contract with them.  These costs are not really related to production, transportation or other operational item.

You may actually have to deal with several smelters before you find one to work with.  This may require trips, meetings, and sending samples of the concentrate to them.  These samples may end up being a ton or more, or at least several pounds.  You will not get any reimbursement for this from the smelter, in fact you may have to pay them to test it.

Aggregating these under the term "marketing cost" reflects their purpose.

Marketing costs are generally captured by the Net Smelter Return calculations, which should include finance costs and freight charges. Additionally you can include General & Administrative costs (G&A): depending on the scale of you output, these costs will range from US$1.00 ~ US$5.00/dry metric ton of concentrate. Depending on the location of your project, it is not too difficult to reasonably estimate both NSR and G&A costs.