I have a custom PVC cage in a room that can get as low as 55 degrees, and I use an 80-watt RBI panel on the cool side, with a UTH to make a hot spot. The hot spot is on the side without the RHP, with my logic being that in addition to the higher surface temperature, it helps keep the corners from getting quite as cold. I designed a bunch of features of my cage around retaining heat as well as possible, and it is a non-standard size, but in general it is in the same ballpark size-wise as some of the others mentioned in this thread.

The big thing to remember about heating cages in a cold room though is INSULATION, even for PVC. If your RHP is mounted to the ceiling of the cage, a lot of heat is going to be lost through the roof. So especially insulate the top of the cage really well. Also place the entire cage on top of a piece of foam insulation, plywood, etc, especially if you have a UTH underneath. Again, if your cage is sitting on a stand that conducts heat well, such as a metal shelf, more of the heat from your UTH will be escaping into the stand and the room than going into the cage. And adding some amount of insulation to the back and sides doesn't hurt either. It doesn't have to be ugly; I made some insulating panels for mine that look sort of quilt-ish and match the decor in the rest of the room.
And when we're going to be away and the heat is going to be on the "vacation" setting, I put a blanket over the front to cover the sliding doors too. The RHP can still maintain good temps without that, but it helps keep it from having to run quite as high.