The government fees are the same regardless of who reimburses them: port-of-entry admission fees for a border application, or USCIS fees under Form G-1055 for an I-129 petition. Reimbursement is a term of her contract with the agency.
Fees follow the route; reimbursement follows the contract
Hypothetical example: a Rothesay accountant's agency contract says immigration costs are her responsibility, while the client has told her verbally that it covers such things for contractors. She should settle that in writing with the agency before travel, because the agency is the employer and the party writing the letter. The government charges depend on the route: a port-of-entry application involves the fees collected at admission for herself and any TD dependants, while an I-129 petition filed by the agency involves the USCIS petition fee and any optional premium-processing fee, both set by the current fee schedule.
Additional costs might include a credential evaluation if she relies on a foreign degree, certified translations and professional review of the letter. If the placement is extended or she moves to a different client, a new letter and possibly a new application will be needed, so the contract should address who pays for those too.