I asked my OB this when I was pregnant with DD - he said "well if you have the shot you have a lower chance of having a PPH (he quoted me a figure but I can't remember it now) and if you don't have the shot and have a PPH you will need a lot more than the shot" by which I assumed he meant a synto drip which they used to stop the bleeding.
In any case, I had the shot, still had a bleed and needed the drip so I'm absolutely no use! Maybe have a google around for PPH you might find some useful figures on it.