Costs depend entirely on which route, if any, fits the spouse's circumstances, and can include separate legal review, filing fees for the spouse's own application, and possibly credential evaluation. These are additional to, not included in, the principal applicant's TN costs.
Don't assume the spouse's route is included
A quote covering the principal's TN application usually doesn't include a spouse's separate work authorization unless explicitly scoped that way. Ask directly whether the spouse's case is included or priced separately, and get an itemized estimate once the applicable route is identified, since costs vary widely depending on which category, if any, actually fits the spouse's situation. Ask the scope question in plain terms at the first meeting: is the spouse's own position inside this engagement or outside it.
The answer is frequently outside, for good reasons, and it is far better established before a fee is agreed than after a household has assumed otherwise for three months. Ask as well what the initial assessment of the spouse's position would cost on its own, since that is a bounded piece of work and it produces the information the household actually needs: whether a route exists, what it requires, and roughly how long it takes. Everything after that can be decided with real information.