Good question. First judge the gun, not the maker. Most Brit makers offered boxlock guns sourced from Birmingham makers (finished or in the white) with their name and address on them. They were offered for commercial sale by dozens of Birmingham makers and ranged from 'Best' guns to utilitarian 'Keeper's Guns'. Indeed, many London makers best guns were, in fact, made by Birmingham makers like Leonard, Ford, A A Brown, or others. So judge the gun, not the name on the rib.
As for price, it depends on the gun, but you will pay a premium for a subgauge gun, with 28s being even more, dur to demand and rarity. a very good 20 gauge, lightweight boxlock 20 gauge can be bought for as little as $2500. Dealers in the USA will charge more for such a gun than dealers in the UK, often substantially more.I buy them in the UK and have them imported by an import service. This will cost an aditional $800+, depending the the sale value (which effects import duties, and the whim of the importer.
You will find many of these guns for sale on a website 'guntrader' dot UK. lots of drool material there. Of course, you are buying without being able to actually view and handle these guns, so you need to go throug a reputable UK dealer. Very difficult (impossible?) to return such a gun if it doesn't suit you.
I have purchased four or five shotguns this way and have never been disappointed. All but one have been 16 or 20 bore guns and ranged in price from $2250 to $2500 for boxlocks and $6000 to $12,000 for London sidelocks.