WOW! I guess it's a matter of where you're from. I usually pay around $30/tank for boat dives although in my hometown it's $50/tank. I don't dive at home in part because of this (I live on the gulf side of Florida). I drive across the state to the Atlantic or to the Keys where the diving is better and the boats charge around $30/tank. The only time I've paid the prices you're paying was when I dove the Epcot DiveQuest aquarium.
I dive around Hawaii multiple times a year and other places in the Pacific. I still haven't made it to the Caribbean because of family obligations in Hawaii, but even here in Central Texas all the shops push the standard $10/tank rule of thumb (and Hawaii, Guam, Okinawa, California - but those are mostly American ports with a tradition of tipping well). Besides, the shops should be telling customers a higher amount then leaving it to the customers to reward good service (or not reward poor service). It's a rule of thumb not an iron-clad rule.