Cost per dive is a good metric for assessing potential trip value, as long as you factor in the fact it's not the only metric. Like height, weight or BMI for physical assessment of a person, it's a starting point you build on. Here's how I size up a trip.
1.) Consider a destination with the kind of diving I'm looking to do - tropical coral reef, big animal, kelp, etc...
2.) Find a dive op. and accommodations provider and get a package price. Factor in food and tips, if any.
3.) Find out what round trip airfare, baggage fees and other expenses are likely to run.
4.) Consider total trip cost against number of dives.
That's why my trip reports over the years have shifted into giving detailed cost analyses. Many people are budget-limited and it doesn't matter how great a Galapagos live aboard would be if you can't afford to go!
And yet...there's more. From a U.S.A.-based perspective, you might notice some Red Sea liveaboards are cheap, and wonder why many Americans head to the Caribbean instead of the Red Sea or Philippines, which reputedly have 'better' diving.
There's more to trip cost than money. Total trip time and arduousness of the journey (e.g.: multiple flights, very long flights) matter. For some people, getting nearly half-way round the world is a 2 or 3 day (each way) affair, and reaching the Caribbean is 1 day each way. No flight over 4 hours is preferable to a 12 hour flight. Being away from your family 7-days is more tolerable than 15 days (e.g.: if you do a 10-day distant liveaboard because you can't justify the 'travel pain' for just 7).
So, we've got total trip cost in money, travel time and arduousness to weigh against number of dives, and quality of those dives. People brought up dive time - # dives x average dive duration. What about that?
It depends. If you dive the deep offshore wrecks out of Morehead City, North Carolina, you may be looking at NDL-limited dive times around 35 minutes, and maybe 2 dives/day given the transit time to/from the sites, taking up a good portion of the day. But do you value diving authentic wartime sunken wrecks and at times fairly big, scary looking but relatively 'safe' (if you don't do anything stupid) sharks?
These factors can inform a general 'value for cost' sense to help compare destinations.
On that foundation, you can still add factors to discriminate by goal and destination. I went to Jupiter, FL, for close up encounters with Goliath grouper; I went back for close up encounters with lemon and tiger sharks. I went to the Galapagos and dove with scalloped hammerhead sharks and dolphins. Dove out of North Carolina for the reasons already mentioned. California for kelp, sea lions, seals and garibaldi.
I love liveaboards because the 'work flow' of the trip is so straightforward and easy; I love Bonaire for the shore diving freedom that gives me so much independence (though it's a lot more work than a liveaboard).
All this assumes I'm on a solo trip and the topside is food, sleep and surface interval. If I've got non-diving family along needing topside entertainment, that's another level.
In summary, cost-per-dive is a useful metric for comparing similar destinations in a region, and it's one of the factors that help appraise a potential trip's value. But alone it is not enough. Once you settle on a destination, it's a useful metric for weighing options and driving your trip cost down - that's what I did with my last Bonaire trip (e.g.: 2 week stay for 2 week's diving but only paid airfare once, non-oceanfront housing, avoided most sit-down restaurant meals and ate cheap).
