Is there a market? Yes, to an extent. But I don't sell mine. If I found a museum quality tooth (I have seen some teeth sell for more than $2,500), I personally would probably just hold on to it. So for me, I am not diving to sell them, I enjoy finding them. That being said, plenty of people sell some or all of what they find. At least one diver on the boat both days was someone who dives for and sells full time as their primary source of income.
As far as justifying the cost - eh, that's up to each individual. Inshore tooth dives in NC run about 275 a day, offshore runs about 300, and it's a long day being that far offshore. To me, its worth it, and I don't make any money. I have fun as it's a type of diving I really enjoy, and I get the added bonus of taking something one-of-a-kind home with me. It's not an everyday type of thing for me, but definitely worth it to me.
I guess you could look at it this way - I paid almost 700 dollars to go out for two days. I probably did get easily 700 dollars worth of teeth, retail value were I to sit there and sell them individually. But the value to me was the good time I had.
There are other cheaper options for tooth diving, there's the Cooper River (blackwater diving, much cheaper, much longer bottom times) which I haven't done yet, there's also a decent amount of tooth diving off Venice, FL (long bottom times as its 20-30' deep, don't find teeth as big, at least I didnt).
