I won't ask why you're breeding unpapered puppies when the shelters are splitting at the seams, having to euthanize healthy, adoptable dogs just because there isn't any space for them, or mention that large breed dogs have an 76% chance of being given up when they grow to full size, a 88% greater chance (over small dogs) of being given up in tough times and have a 93% chance of being euthanized if taken to a shelter (because no one adopted them). If you do something right and get them Vet checked, vaccinated and wormed,offer a health guarantee and they have no genetic defects, you could probably get $500 to $700.
Purebred English Mastiff puppies are priced between $800 and $1000. Buyers will always ask for a certificate of inspection before making a purchase. You may first need to take your puppies for shots before selling them as this will increase the market price.