DH and BC both have availability ratings that function much the same way. DH uses thrones to represent cash, while BC ignores that and just makes Infamy rolls to acquire items based on their availability.
Apart from perhaps ruling that certain Imperial-oriented or AdMech only items might be more difficult to obtain, there really isn't anything you need to convert, just use the Availability ratings straight out of the DH books.
You will want to skim the weapon qualities sections though - a good number of things have changed since DH, so some of the items may need tweaking.
EDIT: And honestly, the thrones system for cash in DH didn't work very well, thus why it was replaced with Influence in Ascension, Profit Factor in RT, Requisition in Deathwatch, Infamy in Black Crusade, and Requisition again in Only War. Penny pinching in order to have enough cash to buy bullets causes more problems in gameplay than it adds in flavor (Don't believe me? Check out how much a Guardsman makes per month versus how much it costs to buy lho-sticks to feed a habit and still buy las packs to feed his gun, you'll laugh. Then look at how much it costs to feed that heavy bolter you just purchased, you'll cry)