If the prop is windmilling in a steady state at some RPM, it must be absorbing the right amount of energy from the airstream to balance what is needed to turn the engine over at that RPM against the internal friction, compression, pumping etc. If it were true zero friction, it would absorb no power.. if it can't overcome the friction, it will stop.
If the prop is stopped, you're looking at whatever is pretty close to the drag figure for a flat plate of the same projected area at that speed - that should be pretty straightforward to figure. My gut feel says less, in the same way that a stalled wing can lift less than an unstalled one (the prop is just a wing..), but honestly I don't know.
Having actually performed a glide approach with the engine shut down, I can't say the transition from idle power to windmilling, to stopped was noticable - I wasn't looking at the VSI etc., but in terms of seat of pants perception of glide angle, it wasn't evident. It did take a lot of effort to actually stop the prop however (well into stall buffet). I did find it a little disconcerting that the prop suddenly stops being transparent, and in this case (decathlon) with centreline seating, it chose to stop dead vertical, right in front of my nose, no missing it..