Let's see if I can explain this so that it makes sense.

It's sort of complicated & has different facets on how it all works.
The cash prices for additional guests thru DVC are basically set by
DCL. They're not about to lose money on anyone. As you probably already know, DCL has tiered pricing and they raise their prices as time goes on category by category as they sell out. They may not raise ALL categories on the same day - it is dependent on how the categories are selling individually. So when they very first release their itineraries that is THE cheapest time to book and the best deal you will find. Often, travel agencies will buy up blocks of cabins at this first tier pricing - this comes in handy once the itineraries have been for sale for say 6 months and DCL has put out some price increases - you can usually find a better price thru a TA. People THINK the TA is giving them a discount but in reality, they're still just selling cabins that were purchased at a lower fare than what DCL is currently charging. If the TA was out of cabins in the category that you wanted then you would have to pay a higher rate. That's why sometimes people will say they got a Cat 4 thru a TA for less than a Cat 5 directly from DCL - the TA simply still had cabins at the first tier pricing. ANYWAY, thru previous experience DCL knows approx. how much of a % increase they expect to raise a given person (slot) within a category. So they set the DVC cash price at the highest that they expect to go on a normal cash basis from the very beginning. It will not change. So if you're booking on a combination of points & cash the requirement will not change between day #1 and the last day you can book if you book thru DVC.
Real life example: A couple of years ago we were looking to book a DCL with points. In trying to decide on a category, I priced & compared booking a lower category all on points for the 3 of us VERSUS the idea of booking a higher category using a mix of points for DH & myself and paying cash for DS. I compared it to booking all on cash thru DCL so I could get the per person amounts. The cash price for DS's portion as the 3rd person in the room was about $300 higher thru DVC ($1,200) than thru DCL ($900). That's because I was pricing it out in the very, very beginning of the released schedule. DCL expected that the 3rd person fare in that category would go up by about $300 by the sailing date. So they were charging that amount up front via DVC. There was no discount to book early! If I were to check on the same scenario 4 months prior to the sailing date there would not have been a $300 price difference for DS between the two. It would APPEAR to be the same price by then. That's why I said that DCL is not going to lose anything on any pax!!! They just get it up front on this one.
Do you follow that? Does that make sense? In the end, we booked a lower category for all of us on points....however I seriously doubt we'll ever cruise on points again. The point requirements have gone up SO much since we booked that cruise. The same 4 night cruise in July in a Cat. 10 has gone up something like 87 or 89 points or something crazy. IMO that's a lot. We re-booked onboard for the rebooking discount & I suspect that we'll be cash cruisers from now on. Oh, and no, I do not believe you will get credit for a re-booking discount if you re-book onboard and then try to transfer it to DVC.
Finally, taking DVC out of the mix and using a TA to book on cash - yes, you can get an onboard credit with them, but not combing with DVC. If you use anything DVC then you have to book thru DVC. When we re-booked onboard last summer we got whatever onboard credit DCL was giving in addition to the re-booking discount THEN we transfered our ressie to
Dreams Unlimited to get their onboard credit too. So we'll have a 10% discount + at least $400 ship credit. Meanwhile, it's all still just a "dummy reservation" that we're pushing further out. We re-booked for Dec 2009 then when the 2010 itineraries were released we pushed it to Dec 2010. We're actually waiting for 2011 or 2012 tho when the new ships arrive - then we'll decide on our itinerary. We now know that the Wonder is going to be permanently moved to LA in 2011 and the first new ship will take over the land / sea packages but they haven't released the itineraries for the Wonder yet. So we're still playing the waiting game. If we want to go on one of the new ships we'll have to wait for the 2nd one in 2012 as we don't want to do the shorter cruises again. If they release an itinerary we like on the Wonder then maybe we'll do that. It's too early to know.