This will give you a rough estimate:
http://www.bluelight.org/vb/threads/564885
Buprenorphine is never dosed orally (and hydrocodone never IV), so a straight conversion isn't really possible. Taken orally, 30mg hydrocodone is equaivent to 30mg morphine PO, which is equivalent to 10mg morphine IV, and 10mg IV morphine is equivalent to 0.3mg buprenorphine IV.
Now that isn't exactly correct, more a rough estimation. Anyways, that would suggest that is 30mg hydrocodone is equivalent to 30mg morphine orally, and if that is equivalent 10mg morphine IV, which is equivalent to 0.3mg buprenorphine IV... so basically 30mg hydrocodone orally is equivalent to 0.3mg buprenorphine IV.
HOWEVER there are a LOT of caveats to the logic I just used. The least of which is you are (hopefully) using buprenorphine sublingually, so the BA of that would be less than IV. I know from personal experience that 0.5mg intranasal buprenorphine (again it has a slightly higher BA intranasal than sublingual) will keep me good and well (and buzzed) with a 40mg+/day hydrocodone tolerance.
Basically, start low and work your way up to a dose that holds you. Just make sure to start super super low with buprenorphine, because it is a very potent opioid. It's hard to OD on compared to other opioids, but even without getting into life threatening territory taking too much can make you sick for 48+ hours.