i agree with stork320, try scuba world, they have a nice facility just beside the hilton hotel and condominium, there are instructors there that could give you a little bit of discount if you are a filipino, range between 10K to 15K. i think thats the standard rate nowadays for most shops, you can dive a lot cheaper if you are already certified and more so if you dive in a group, usually its the boats that are expensive especially when you want to dive in the islands adjacent to cebu that's why i suggest you dive in a group to minimize your expenses.
but since your gf is just new to diving then you can always opt for beach entry, there are places in mactan where you can do this without extra payment as long that you have your gears, you can also rent gears if you knew someone in the shop and just bring it to where you want to dive.
i suggest you try the marigondon cave just in front of plantation bay mactan, its a very good spot especially if our into cave and cavern diving, but then its not for the beginners. but hey, just tell me what's your preferrable price range for your diving, maybe i can inquire with my friends in here.