From my experience it is not more expensive to book directly with
DCL, but you have a much smaller chance of getting any on board credit.
I booked directly with DCL when I saw both the cabin and dinner time I wanted were available. I didn't want to look for a TA and risk my cabin disappearing in the process.
I've actually been quite happy with my booking. It's been really easy to make payments and additions to my booking whenever I want.
However, there was a stage where I looked at transferring my booking to a US Disney specialist to get the OBC. Disney have made it harder to do this and my OBC was going to be considerably less than had I booked with the TA right from he start. I emailed the TA a few other questions about their handling of WDW reservations and I wasn't at all impressed by their generic uninformative responses. (Basically, reading between the lines, they said 'Sign your booking over to us and then we'll take time to answer your questions properly). They also took ages to get back to me. I couldn't shake the feeling that approx $100 of OBC wasn't worth loosing control over my booking and risking the potential frustration of bad service.
So sorry, this doesn't really answer your question, other than to point out that if you want to use a TA, don't book with Disney first. You'll get less OBC.