That is a commercial term to settle in the engagement, not an immigration rule. The government charge depends on the admission route and is published in the USCIS fee schedule; professional fees, a good-standing letter and travel are separate items.
Putting it in the engagement
Write the cost allocation into the secondment agreement. Typical items are the government charge for the port-of-entry application or the I-129 petition, any premium-processing charge if the petition route is used, counsel's fee for reviewing the structure and drafting or correcting the U.S. letter, a good-standing confirmation from the provincial order, and her travel and accommodation.
Ask counsel to quote the structural review separately from the application, because the household may want the first before deciding on the second. A quote that excludes responding to questions from the port of entry or USCIS is incomplete. The firm should also consider the cost of covering her Québec clients while she is away, which is not an immigration expense but is the real cost of the secondment.
Confirm the current government amounts on the fee schedule in the week of filing, and keep the household's costs, the partner's franchise budget and the children's school fees on separate sheets since none affects eligibility.