The published costs are the Form I-129 filing fee with its L-classification supplement, any applicable additional fees, and optional premium processing, all set out in the USCIS fee schedule, Form G-1055. The larger and less predictable expense is producing the corporate evidence that a small company has never previously had to assemble.
The evidence costs more than the filing
For a company that has always been small and privately held, the petition forces work that has nothing to do with immigration. Ownership may need to be reconstructed from years of informal arrangements. Financial statements may need to be prepared to a standard the business has never needed.
Organisational charts may have to be drawn for the first time, and drawn honestly. Hypothetical example: a welding and equipment-fabrication shop in Sainte-Cécile-de-Milton is weighing whether to commission a compilation report from its accountant showing that both it and its Ohio counterpart are actively trading. The report costs money the owner would rather spend elsewhere.
Against that sits the cost of a request for evidence months later, the delay it causes, and the possibility of a refusal that any future filing will have to explain. Buying the evidence at the outset is usually the cheaper of the two.