Government fees vary by route and are published in the USCIS fee schedule. Beyond that, the real costs are credential evaluation where required, certified translation and professional advice on the duties description.
Spend on the facts, not on volume
The two routes carry different costs: applying at a port of entry as a Canadian citizen is not the same transaction as an employer filing an I-129 petition, and the employer usually decides which happens. Check current amounts against the published USCIS fee schedule rather than any figure quoted in conversation. The discretionary spending that actually changes outcomes is narrow. A reliable foreign-credential evaluation is worth paying for when the regulation requires one. A certified translation is worth paying for when documents are not in English. Advice on how the duties should be described is worth paying for once, early, before the letter is issued. Paying for a thicker file does not compensate for an activity that falls outside the listing.
Hypothetical example: a cartographic consultant accepts a short project and pays for an extensive advocacy package before obtaining the employer's final duties. Start with a budget that separates unavoidable government charges, a required credential evaluation, translations, travel, and optional professional help. The first review should produce a go/no-go list: which facts must be confirmed before any discretionary expense, and which records the employer can provide without cost. If the duties do not match the listed profession, paying for more pages will not repair that problem. A modest, accurate file usually requires less rework than a costly file built before the assignment is defined.