That is a matter for negotiation with the employer. Government charges follow the USCIS fee schedule, Form G-1055, where the employer files Form I-129, or are collected at the port of entry for a direct application. Document costs, travel and any professional fees fall wherever the parties agree, so agree it before the offer is accepted.
Settle who pays while you still have leverage
The moment to ask is during the offer discussion, when the company wants the candidate. Ask three things: whether it will file an advance petition or expect a border application, who pays the government charges in either case, and whether it will cover the cost of certified transcripts, travel to the port of entry and any professional fees. Also ask what happens if the application is refused, since the applicant may have resigned a position in Ontario by then. None of this is unusual to raise with an employer that has hired across the border before, and an employer that reacts badly to the questions has told the applicant something useful. Confirm the current government amounts at the time rather than relying on a colleague's recollection, as fees are revised periodically.
Hypothetical example: a soil-salinity mapping specialist receives an offer from a start-up that has never hired across the border. The first review asks for a written allocation of government charges, petition preparation, credential retrieval, travel and any response to an agency request. The offer letter or reimbursement policy settles who bears those items; a verbal promise from a hiring manager may not survive a budget change. Ask whether work can start only after approval and what happens if the company changes the role. Confirm official fees at payment time because published amounts can change.