Hi, Sharkman!
It has been awhile since I went to Bonaire, but I think the following comments are still valid.
Not surprisingly, it depends on what you do and where you stay. You can do a real cheapy on Coz, if you find a single room somewhere or if you stay in one of the very inexpensive "residence hotels" downtown. I have known folks who just hop a plane to Coz, look around for a room and go diving. The only reservations they made are for the airplane and the diving. Rooms have been known to go for as little as $25.00 per night. But if you stay in a "regular" hotel, Coz and Bonaire are about comparable.
Airfare used to be more expensive if you went to Bonaire, but the rates to Coz have risen lately and, so, that may no longer be true.
However, the diving was usually where any higher costs on Bonaire compared to Coz evened things out. If you did mostly shore diving on Bonaire (and most folks do since it is so great and the sites are so well marked), it was much cheaper per dive than boat diving on Coz.
So, there you are: it is probably still a push these days.
The reason I would chose Coz over Bonaire are purely personal:
(1) Coz is a shorter plane ride and, if you fly Continental, much more reliable than the plane rides to Bonaire, especially the final leg.
(2) I happen to like the diving on Coz better, but that is just me!
(3) I happen to REALLY prefer live boat drift diving--and that is what you have on Coz.
(4) I like shore dives, but, given a choice, I prefer boat dives. There is boat diving on Bonaire, but now the price differential normally shows up--at least, in my experience.
All that said, I strongly recommend that you experience Bonaire because it is wonderful diving. The system at Buddy's, if you choose them, is very diver friendly, too. This is one scuba diver's ticket that should be punched!
Scorpionfish