I don't know that DCU is actually funding these shows. In the recent Office Hours thread, their community manager said that DCU didn't really have anyone giving notes during production; that was all WBTV. So it almost seems like WB was using DCU to showcase shows they wanted to resell elsewhere.
So, my point there is, the CW exclusive may limit the upside on the show going forward, which could, in fact, limit the budget. OTOH, Stargirl has been an extremely good performer for the CW, so maybe they'll look more kindly upon it.