Government charges depend on the route chosen and are published in the USCIS fee schedule and by CBP; they change, so check them on the day you file. The private costs are usually document gathering, certified copies and professional review.
Route choice drives the government cost
Hypothetical example: a Grand Bay-Westfield applicant budgets from a figure a colleague quoted two years ago and is surprised twice, once by the amount and once by which party pays it. Requesting admission at a port of entry and having the employer file Form I-129 carry different charges, and an employer that files may also pay for optional faster adjudication. Rather than relying on a remembered number, read the current USCIS fee schedule and confirm the port-of-entry charge with CBP before committing to a date.
The costs applicants regularly forget are the private ones: certified copies of the commission, a letter of good standing, courier charges, and time spent by a professional reviewing the employer's letter before it is signed. A letter corrected before it is issued is far cheaper than a refused application and a rebooked start date.