At a port of entry the government charge is the fee published in the USCIS fee schedule for a TN admission, and a petition filed by the employer carries the I-129 fee from the same schedule. The forgotten costs are the second border trip after a refusal and the unpaid weeks after a premature resignation.
Budget for the mistake you are trying to avoid
Government fees are modest compared with the cost of getting the sequence wrong. Refer to the USCIS fee schedule for the current amounts rather than an older quotation; the I-129 fee and any premium-processing fee apply only if the employer petitions from inside the United States, and the port-of-entry route uses a different, lower charge. Professional fees cover reviewing the letter, matching the credential to the Appendix 2 entry and preparing the applicant for the interview.
The expenses households forget are the ones caused by errors: a refused admission means the applicant returns to Guelph/Eramosa, the employer rewrites the letter, and a second trip follows; a resignation given before the decision means income stops for that period. A spouse's state nursing licence, examination and credential-verification fees belong in the same budget, because her income will not start until they are paid and processed.