@splash8
Good to hear your teens are enjoying diving. A family that dives together is a great thing...
Our daughter was certified at age 12, and our son was certified two years later, also at age 12. They are now 21 and 19, and have been diving several times a year since their original certifications (along with getting their advanced and rescue certifications).
From the moment our daughter was certified, we've been traveling with them for diving in the Pacific and the Coral Triangle. The Caribbean is easy, but not diverse, and way too crowded for our taste.
Places we've been that were "reasonably priced..." - Hawaii (we like the diving on the Big Island the most), Yap (as mentioned above, you will see mantas, and mostly likely quite a few), Pohnpei (a bit wild, but very worth it), Fiji (there are some very cheap resorts in FIji, just have to decide how much you want to spend). Take a look at Matana Beach resort - authentic, very comfortable, very good diving.
Tahiti and the Maldives - beautiful, great diving, but not "reasonably priced" (but we've returned to both several times because the diving is that good).
Much of Indonesia (including Raja Ampat) can be done very cost effectively - it's just a bit costly to get there (given airfare, etc.). We've done Raja Ampat twice with our kids, they both really enjoyed it. We've going back to Lembeh (central Indonesia) this summer for our 4th time. We have found Indonesia to be cheaper than most of the Caribbean for hotel, food, diving with airfare being the more expensive bit.
The flights are long from the US, (no getting around this), however we always stop in Japan, Korea, or Taiwan (coming from the west coast) get off the plane walk around the aiport, etc. Then we give ourselves a day in Singapore to get over some of the jetlag, eat good food, etc.
A place you may not have thought of: the Solomon Islands. We've been twice, and my son and I are going back in August. These islands sit just on the edge of the Coral Triangle (so great diversity of marine life), while not being quite as far as Indonesia (you can transit in Fiji or Australia). Check out Uepi Island Resort (their website is being renovated, but they are open). This was some of the best diving we've done anywhere in the world (sharks, soft coral, critters) and the resort is run by an Australian family who's been living there for ~30 years.
The other thing I would mention here is that our kids are so much more aware of the wider world because of our travels - I wouldn't trade this for anything. Our son has played soccer in a local village in Indonesia, and our daughter helped teach an English class for a few days in the Solomons. These aren't experiences that you can put a price tag on.
