The OBC for an onboard booking is $200 for a 7 night or longer cruise. I don't believe that will be affected by transferring to a TA, since it's from
DCL.
Generally, TAs offer some OBC if you use them to book a cruise. They give that OBC out of their "cut" from DCL for booking clients on one of their cruises. DCL has cut the commission to TAs on transferred bookings (since they don't do the initial work booking the cruise), and thus some TAs have also cut the amount of OBC they will in turn give you out of their commission.