As Daryl said "Tough, for an existing rifle." The beautiful very light doubles shown above all look to be somewhat expensive for a feral hog rifle.

Size and weight and caliber aside, this Beretta .45-70 is a very effective wild hog rifle that was made by a former member of this forum. Even slow 300 grain.458 bullets make big, fatal holes in game:

However, it is not for sale and if for sale it would be several thousand $.

So, if I may be so bold as to make a suggestion, have you considered trying an inexpensive larger and more common caliber double to try to see if you really like hunting hogs with a DR? If so, there are several options, but probably the least expensive is a .45-70 Baikal. For example, here is a link to one recently acquired by an Internet contact in Austria:
https://www.africahunting.com/threads/enter-the-minefield-of-double-rifles-from-baikal.55107/

Have fun and please post about your experience.