Art market professional’s concerned about whether the royalty accounts for differing AMP costs should note that the royalty is calculated exclusive of (i.e. disregards) any costs associated with the resale.
This leaves it up to the art market professional, the buyer, and the seller, to freely agree their costs, premiums, commissions etc and determine what is, in their particular circumstances, fair and lawful between them.
It is not the purpose or function of the legislation to presume, dictate, or discriminate those private commercial arrangements. Instead, the legislation creates a predictable royalty by using a fixed percentage of the resale value. This is the same for every qualifying resale, and every liable AMP.
This creates benefits to art market professional and artists alike. Negotiation, custom, and freedom of contract, will determine fairness between parties to the resale contract as to their own costs, premiums etc. And whatever they freely decide about those (and however that varies case-by-case), won’t prejudice or affect the “resale value” or the royalty total, which also ensures transparency and fairness to the artist.