Chip,
Diving around Mactan is pretty good, a lot of wall dives. Go to Hilutungan and Nalusuan, both are marine sanctuary. If you have some time, I would suggest going to Moalboal. Its about 3 hours from Mactan by car. There you can do unlimited diving for $50/day and diving is much better than Mactan.
As for the dive shop in Shangri-la, dont go there, they're way overpriced. The average prices in Mactan is around $25/dive, that includes boat, guide and equipment. Here's a link to some of the dive shops in Cebu,
http://www.wayblima.com/cebu-activities-diving.html. The cheapest one is Liquid Asset for $20 for 2 dives with a minimum of 2 people. I'll be diving with them when I get there. In my case, I'll be dicing my equipment and all I need is a boat and guide, thus I can be cheap
However, if you dont, the cheapest dive shop may not be the best. Having said that, I'm not saying Liquid Asset has old equipment, havent seen what they've got. So, if you dont own your equipment, I would suggest Scubaworld in Hadsan. I've used them in the past and their equipments are comprable with US dive shops, very well maintained. Hope this helps.
Enjoy your stay in Mactan.
Enrique