I saw a 470 built on a Browning SXS action at a gunshow in Vancouver BC awhile ago. I think it was around $4000 US. Too be honest I would not buy a $4000 bigbore Double without knowing all about it and Doubles in particular.

You may find a 400 or smaller for $4000 if you are really lucky and willing to take an odball caliber.

Do you reload?