The petitioning employer pays the USCIS fees for Form I-129 with the L supplement, and premium processing if elected. Amounts are in the USCIS fee schedule, Form G-1055. Documenting the proprietary system is usually the larger internal cost.
Most of the spend happens before the form is signed
Government fees are published and should be read from the G-1055 rather than quoted from memory, since they change. The costs that surprise companies are internal. Producing a coherent record of a system that was built informally over four years takes staff time, and it competes with the operating season.
Legal review of the corporate relationship, and of any third-party management contract, is a second real line. If a request for evidence arrives on the specialized-knowledge question, responding well is expensive. Set against that, an approved petition also carries the cost of the family's own applications and any translations.
The company should agree in writing what it funds, including whether it will pay for Fabien's dependants, because L-2 applications are frequently overlooked in a transfer budget.