As an exercise..I did a wee guesstimate on an image I saw recently..from an amateur's perspective of course.......taken by our beloved Alex Mustard.....it was a picture of a clown fish on an anemone in the red sea.
I asked myself...if I wanted to take that same picture in the morning...how much would I need to spend...with the same kit that Alex used??
A rough guestimate for the same kit.......ie. strobes, camera, lens, housing........about 8000 sterling.
Then get to the Red Sea.... on a good liveaboard....say about 1000 sterling max......
But I cannot forget all the hours spent underwater, and the cash needed to do this......the learning...the training......the mistakes..... the triumphs....more thousands.........
Jeez....this underwater photography is a potentially expensive lark period....and an awful lot of cash goes into getting you....the photographer.... into a position to trigger that shutter.......
I am guessing this is the photo:

It is certainly my favourite Red Sea image.
From an amateur/hobbyist perspective I think your numbers are correct, Damo. But doing underwater photography for a living means controlling this expenditure. No business model would work in this game if you paid out for all photo ops. I have to operate in a way that gives me the chance to shoot, without the expenditure. Which usually means going where people want me to go, rather than choosing where I want to go.
I think revealing a little of what this photo did cost, gives quite large insights in how I find I must operate to make ends meet.
It was taken on my November 2008 Red Sea workshop. The main aim of that trip was teaching, so it cost me nothing to go and I was paid well for going there for the week. Of course, this means that my own shots were a low priority, and I think anyone who has been on one of my workshops would say that I work very hard for the group (on talks, giving advice on the conditions/site/opportunities and on reviewing images). So it is not an easy way to operate, but it does mean I was well into profit for this trip before I took this picture.
I also used this trip to review the new Nikon D700 in a Subal housing. Both of which I had been lent. So again no cost. The lens was a 16mm fisheye, which I had owned since film days, and Kenko 1.5 TC. Strobes were Subtrononic Alphas, one owned from film days, one bought secondhand. So cost per dive/photo/successful photo on these items is now very low. The trip also allowed me to complete the review of this system, although it ended up costing me - because a month later I bought one!!
I think all this is worth mentioning because it gives an insight into the difference between doing this for a living and for fun. uUnderwater photography for a living is all about reducing overheads. The photo you chose randomly was taken on a trip I was paid to go on, with a borrowed camera. Sure I would have rather been in the Galapagos with a D3x, but to make the finances work I don't always get to go where I want.
The hard part is not taking the good pictures or selling them. The difficulty is finding the different ways of funding time in the field to take them, while ensure you remain in profit. It is a challenge of being in the blue, while staying in the black! :-)
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So to get back on topic, I think you have to decide if UW photo is a hobby, which you enjoy spending you money to do and when pictures make money you treat the extra income as a treat! Or whether you are operating from a business perspective. For most people it makes more sense to use their talents to earn money in a proper job, for which they are well paid, and then spend this money enjoying their hobby. I enjoy spending my money on my (dry) hobbies!
Alex