"Best" is a standard of build and materials, rather than a price point. "Best" is the very finest of materials and workmanship embodied in a gun where price is no object; in a practical sense though, there may be anything like 500-1,000 skilled artisans' hours represented in a best; they are priced to at least reflect that.
"Best" often implies a hammerless sidelock ejector of British (often London) origin, in a style perfected in the last quarter of the 19th century, but a maker in any time and place who is sufficiently skilled and experienced who chooses to build a gun (or coordinate a number of specialist artisans in the the building of such a gun) to such perfect standards of materials, fit and finish - and design - can rightly claim to have built a "best".

A new best from one of the leading British makers may leave little change from GBP 50,000-100,000 if it is ordered cased with all the appropriate accessories.
RG