Let me see... as a DM, I get paid nothing for pool sessions, but I don't have to pay for the air or water. For checkout trips, I pay cost, which is usually a nice discount (but it's still paying, not being paid). So, basically, at least here in Baton Rouge, being a divemaster is strictly a volunteer position with a few discounts.
For me, personally, however, I have decided that when I eventually become a full Instructor, I will give *something* to any divemaster assisting me. It won't be an even split, and it's almost certainly not going to be "good" pay, but *some* gratuity in some fashion seems only honorable to me. Even something as simple as picking up a lunch tab would at least show gratitude. A good DM in my area is certainly not doing it for the money, but that's no reason not to thank them with something at least slightly more weighty than words.
(For the record, it doesn't bother me that I work classes as a volunteer. I do it because I enjoy it. I just want to be a person of above average consideration -- we could use a few more of those around.)