The employer's time is usually the main cost, along with legal review of the draft. Rewriting a letter before it is used is less work than responding to questions raised by a vague one. Government fees depend on the route and should be confirmed directly.
Spend the effort before the letter is used
A short review cycle, where counsel marks what is missing, the employer confirms the facts, and the letter is reissued, is inexpensive compared with rebuilding a case after an officer's questions. Where the employer resists, ask which specific statement they are unwilling to make and why, since that answer often reveals a genuine disagreement about the role that needs resolving anyway.
The government charge depends on the route chosen, a port-of-entry application or a USCIS Form I-129 petition, and current amounts are published in the USCIS fee schedule. Create separate columns for compensation, employer business expenses, professional-preparation charges, and government filing or admission payments. Canadians may generally request TN admission at a port of entry, while a US employer can use Form I-129 when suitable; those routes have different official charges and preparation needs, which should be checked against current government instructions.