What is the average price for a bull mastiff?

I'd check out petfinder.com or craigslist (under the pets section). We've rescued all of our mastiffs from there for under $300 (full bred Dougue Bordeaux and English Mastiffs as well as Cane Corso) Sometimes it takes a little while to find your perfect puppy/dog, but I highly recommend these websites. Not only is it cheaper but you're also saving their lives from kill shelters and puppy mills! Good luck!