A gift to the U.S. government in the published amount and a processing fee, at the stages the official page describes. Private adviser fees, relocation costs and the TN process are separate and outside the program.
Three columns: gift, processing fee, everything else
The official framework lists a gift amount for an individual applicant and a different amount for a corporate sponsor, plus a processing fee; the page describes when each is paid. The gift is not an investment, is not placed at risk in an enterprise, does not create jobs and should not be expected back; the processing fee is a charge for handling the application. Everything else the couple might spend is outside the program: advice on reading the terms, translations, travel, housing, and the separate costs of the nurse's TN application, which follow the USCIS or port-of-entry fees rather than anything in the Gold Card framework.
Keep a written budget in three columns so that a private fee is never mistaken for a program requirement, and so the couple can see the total before any payment. Confirm the amounts on the official page on the day of payment, since a change in the published terms changes the budget.