Broadly the cost of doing it again: preparing the documents, seeking a further admission or filing a petition, and the professional time to assess whether the change is material. Government charges should be confirmed on the current USCIS fee schedule. Travel and lost engagement days are usually the larger figure.
Budget for repetition, because change means repetition
Consultants tend to price the first admission and treat it as the whole immigration cost. In practice a consulting career in the United States involves the process repeatedly: each new client, each material change of scope, each extension. The recurring components are advice on whether the change is material, preparation of the new agreement and support letter, and either a further presentation at a port of entry or an employer petition, with charges confirmed against the current published schedule.
Around them sit costs that never appear on an invoice: days not billed while travelling, a client whose start date slips, and the risk of an unfavourable decision at the border partway through an engagement. Where a client is willing to file a petition instead, the timing is more predictable and the cost profile changes. It is worth asking the client which it prefers at the outset of every engagement rather than assuming the last approach still applies.