Preparation of the amendment, the applicable filing charges taken from the current USCIS fee schedule, and corporate legal work to establish what the transaction did to the qualifying relationship. Where an extension falls in the same period, sequencing the two can avoid duplicating the evidence and the fees.
Amendments are cheaper than the alternative
There are really two figures to compare. The first is the cost of doing the amendment: professional time to assess materiality, prepare the filing and assemble updated corporate evidence, plus the applicable government charges, which should be confirmed on the current published schedule because L filings can attract more than one. The second is the cost of not doing it, which is harder to quantify and considerably larger.
It includes an extension refused because the current facts do not match the approved ones, a period during which the individual's status is questionable, and the disruption of a family that has to leave at short notice. Employers weighing the first figure should also count the corporate legal review of any purchase agreement, which is needed for reasons beyond immigration anyway. Where an amendment and an extension are both due within a year, filing them together often reduces the total, and counsel can advise on sequencing.