The government charges are set by the USCIS fee schedule, Form G-1055, for an I-129 filing, or the port-of-entry application and I-94 fees collected by CBP for a direct application. Beyond those, budget for credential documents, any professional preparation and the applicant's own travel; an employer is not required to reimburse a port-of-entry applicant.
Separate government fees from preparation costs
Build the budget in three columns. The first is government fees: consult the current G-1055 schedule for the I-129 route, or the CBP charges at a port of entry, and do not rely on a figure from a colleague's application two years ago. The second is document costs: certified transcripts, replacement certification cards, and any translation if a credential was issued in another language.
The third is professional preparation, which for a technologist is mostly the time spent getting the employer's letter right. Ask the employer up front which of these it will cover and whether it prefers the I-129 route, which shifts the filing fee to the company, or a port-of-entry application, where the applicant pays on the day. Neither route buys a better outcome; the choice is about who wants an answer before the applicant travels.
Keep receipts, since a later extension file often repeats the same documents.