Because some consultancies bill the method as a separate deliverable, often with an "international expertise" surcharge. For an SME with 80 employees, we saw a "GHG Protocol" quote at €28,000 excl. VAT alongside a "Bilan Carbone®" quote at €13,500 excl. VAT for exactly the same boundaries. Where the difference is justified, it comes from elsewhere: company size and complexity, scope 3 detail, the provider's profile and additional deliverables such as an action plan or SBTi support. These are the points to compare between 2 quotes, rather than the method's name.
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