Model the household on the assumption that the second income starts later than hoped. Beyond the US$15,000 fee and the gift for each applicant, other visa, medical, and professional costs apply, and permanent residents are taxed on worldwide income under ordinary US rules. Budget for the period before permanent residence or another valid work authorization is obtained; admission as a permanent resident itself generally permits employment.
Price the delay, not just the process
Write out the household's monthly costs after arrival and multiply by a conservative number of months before a second salary appears. Add professional fees for the immigration work and for tax advice, since worldwide income reporting under permanent resident rules may change what a foreign salary or bonus is worth. Then check whether the plan still holds if the spouse's employment cannot begin as early as the couple currently expects.
Two further items belong in that model. The first is tax advice taken before anyone moves, since lawful permanent residents are taxed on worldwide income under ordinary United States rules and an asset structure built for a Canadian resident may need attention while choices remain open. The second is the cost of maintaining the spouse's professional standing through a pause: registration fees, continuing education requirements and, in some fields, a minimum of recent practice.
Those are small annual amounts that preserve a career, and they are among the easiest things to cancel and the hardest to restore.