The cost is mostly the professional time to review the engagement documents and, where a restructure happens, the employer's expense of adding a payroll position. Government fees depend on which filing or entry route is eventually used, and those amounts should be read from the current USCIS fee schedule rather than quoted from memory.
Price the review, not the outcome you hope for
Ask an adviser to scope two things separately: the assessment of the arrangement as it stands, and the work involved if it is rebuilt as employment. Those are different pieces of work and it is reasonable to be quoted for them separately. On the employer side, moving someone from an invoice to a payroll line has its own cost and administrative consequences, which is why some companies resist it, so raise it early rather than at signature. Be cautious about any quote presented as a single all-inclusive figure before anyone has seen the ownership documents, because the ownership question is exactly the sort of fact that changes the scope of the work. Keep your own record of what was quoted, for what, and on what assumptions.
Hypothetical example: Gideon, an actuary, receives a contractor quote that excludes the employer’s payroll conversion costs. He should first decide whether the proposed relationship will actually be employer-directed, because TN does not permit him to operate his own U.S. business. The budget can then separate credential records, translations, professional review, travel and any I-129 option from the commercial cost of revising the contract and payroll. Government charges should come from the current USCIS fee schedule rather than a recycled estimate. A low quote built on independent-contractor assumptions is not comparable with one based on compliant employment, so the engagement terms must be fixed before price becomes the deciding factor.