Nothing in the TN rules requires the employer to pay, so the offer letter or employment contract decides it. The government charges are the CBP fee at the port of entry, or the USCIS petition fee on the current G-1055 schedule if the employer files Form I-129, plus any premium processing the employer chooses.
Read the offer for who bears each fee
Hypothetical example: a Haldimand County CPA working for a farm-supply cooperative is offered a controller position by a U.S. grain merchandiser, and the offer is silent on immigration costs. Accountant is listed with a baccalaureate or CPA credential rule, so eligibility is not the issue; cost allocation is.
The port-of-entry route involves the CBP admission fee, credential evaluations if any degree is foreign, and professional fees for preparing the letter. The petition route adds the USCIS I-129 fee and, optionally, premium processing, both on the current fee schedule rather than any figure remembered from a previous job. Budget separately for the state CPA licence or reciprocity if the role requires signing as a CPA.
Ask the employer in writing which items it covers, whether it reimburses on approval or pays upfront, and what happens if the application is refused. An offer that shifts every cost to the employee is legal but should be priced into the salary comparison.