Repeated travel costs, and any fee that applies when a new TN admission is requested rather than returning within an existing admission period, are the main additions. Confirm current fee amounts directly with the relevant agency, since they change and are not set by the travel pattern.
Separate travel spend from application spend
The recurring cost of flights or drives is predictable and easy to budget. Less obvious is how often the employer will need to reissue documents, or how often a new TN admission may be sought over a long engagement, each of which carries its own cost in time and fees. Agree in advance who pays for reissued letters and any credential documents that need replacing.
Build separate cost columns for obtaining academic evidence, any foreign-credential evaluation, a required professional licence, document delivery, advice and the chosen government process. Once the process is chosen, check current government charges in the USCIS Form G-1055 schedule or the applicable official border guidance instead of relying on an old estimate.