Those are custom made Hunt Breeches, definately not something you would see in the H/J world. Bespoke tailoring from Saville Road in London is VERY expensive. I'm not a Hunt person, but maybe those are more correct for the Hunt Ball rather than riding to the hounds.
True, but it's expensive (in part) because it is specifically tailored for that customer. What if it was tailored for someone with really odd proportions -- someone who HAD to get custom-made clothes?
(Kind of like the Ariat breeches I once bought on-line, that seemed to be made for a 4 foot tall person with 30" waist but enormous belly.)
You could get them made for a lot less money - just not on Saville Road in London. Using the fabric and design elements normally used for breeches, a typical tailor could make a pair of breeches for substantially less than those in the listing.