It took a lot of rounds of gunner ammo per Luftwaffe plane shot down, but that is not the most-important statistic. More important is how many Luftwaffe planes were shot down by B-17 and B-24 gunners. (Keep in mind, there were slightly more B-24's in WWII than B-17's -- it's not only about B-17's.) Also important is that Luftwaffe fighters didn't like to hang around among the bombers because of how heavily gunned they were. It changed their tactics and their time of fire on bombers, which was very, very important.
An example of what LW pilots thought of this. From "Fips" Phillips, a 200+ Eastern Front Ace wrote the following while in command of JG 1 defending against American Bombers over Northern Germany: "Against 20 Russians trying to shoot you down or even 20 Spitfires, it can be exciting, even fun. But curve in towards 40 fortresses and all your past sins flash before your eyes."
I doubt very much that having gunners on B-17's and B-24's were not a major factor.