A new BC will cost $325 to $350, the hard part will be finding one!! RR had 15 of em 3 weeks ago, they're out now.
The BC uses the same SB2 frame as the .500 S&W Handi which has a SAAMI MAP of 60kpsi which is 10kpsi over Ruger data. How long the underlug would last at Ruger levels is another issue, but +P loads are levergun levels, so they wouldn't be an issue, H&R will tell you to shoot levergun loads in it. Both Buffalo Bore and Garrett ammo are safe in the NEF Handirifle as stated on their sites, you'll find that Garrett doesn't list the NEF in their heavier loads, that's because the Handi and BC .45-70s have short throats in general and you won't be able to load em.
Tim
http://rrarms.com/catalog.php?prod=GCR1871