If it is a genuine broker, I would presume that they are working with a prospective buyer. I don't think they would think in formula terms like you mention, as a true broker is not buying to resell, but rather they are representing either a prospective buyer or a prospective seller.
If acting as a buyer broker, they have probably discussed with the potential client both their budget and comparable name sales to give an estimate of the value. It is likely the first offer leaves some room for negotiation, as others have noted above. But it is probably within the ballpark of what they honestly believe the name is worth.
While not every broker in world is mentioned, Elliot Silver,, has a list of domain brokers here.
You would have the option, if it is a valuable name with many potential buyers, of contacting a seller broker who would suggest a price range to you. If you do that, however, be upfront that you have already been approached by a buyer broker, and their offer. Sometimes two brokers work together to complete a deal. Of course each broker would need to be compensated, so having more than one may make the deal less likely to close.
Since it is a domain name already in use, I think the ultimate question is how much is the name worth to you, do you want to consider selling it, and is the return enough to cover the hassle it would be for you to abandon the name.
I presume you have already queried NameBio to and GD Appraisal to find similar domain names in the past, and queried the main brandable marketplaces to check out pricing on similar names listed for sale.
Best wishes however it turns out.
Best wishes.