Some young folk are running a dive shop in Mazunte. They offer a "Discover Scuba, " for 1200 pesos. If you are a certified diver, it is 1200 pesos for a two tank boat dive. They don't have many tanks nor regulators, so the only rentals offered are of snorkeling gear
I also visited with Gener, who is running Azul Profundo out of Cordelia's on Playa Panteon. He'll do a 30 meter boat dive for 700 pesos. But these are serious tech divers/fish hunters. He will try to talk you into the 50 meter hunting ecpedition for a thousand pesos. I hope you know your personal limits.
He will rent tanks and weights. Prices are going to vary.
Beto's still up that damned hill somewhere. But I 'spect you'd need an intro from the right person to lay hands on his gear.
I know this sounds like nailing jello to the wall, but such is diving putside the major resorts on this coast. Sigh.