mastiffs are very good at noticing anything out of the ordinary, so coupled with a german shepard would be a good guard/protection dog couple.
Actually the food bill for both dogs, the Dane and my Standard Poodle, ( Another really great dog, super smart easy to train and every bit as watchful as the Dane) is about 150.00 / month.
True, Danes unfortunately aren't generally long lived 7-9 yrs, I do have a buddy and his old Dane girl is 11, but she is showing her age.
Still all in all Danes make a great dog she sleeps lots and most of the time on the sofa as a couch potato. But at least once a day she has to run, we go out every day and she tears up the field next door,watching her run is poetry in motion.
I remember those attacks in SF some years back. Those were presa's that were horribly inbred by some people using breeding as a front for some people locked up or some shit. Gimme a few and I'll see if I can find links.
But Bobby's right Presa's and Cane Corso's are in the Mastiff family, but a bit different than the English or Bull Mastiff that most are used to.
They are closer to bulldogs than mastiffs, but bulldogs are considered in the mastiff family as well. Corso's and Presa's are very similar. IMHO same dog, different location,(one Canary Islands, one Italy) lines taken in similar directions. One the butcher's dog, and the presa for guarding livestock. Both very dominant usually as Bobby stated.
To answer your question though, yes they make excellent guard/protection dogs but are kinda challenging for some as they are powerful breeds and they tend to know it. Also do tons of research and look for breeders with verifiable working lines for best results. Good luck on your search.