The professional ones. Credential assessment, state licensing and examination fees are usually larger and earlier than the government charges published in the USCIS fee schedule.
The expensive part is not the application
Households budget for the immigration step and are then caught by everything sitting in front of it. An equivalency assessment of a degree, a state licensing application, an examination with limited sittings and any travel it requires, and professional advice all fall due before the application itself and are generally non-refundable. Government fees vary with the route chosen and should be read from the official fee schedule rather than from a summary, since amounts and options change.
The question worth asking early is which of these costs become committed at which point, so the household can see when it has effectively decided to proceed. A Douro-Dummer applicant who maps that out usually finds one or two decision points that were invisible before, and can choose to reach them deliberately.