For a dive course just a 20$, bottle of wine, some chocolate and etc. is a very nice way to say thank you and that you appreciated the work and patience.
DM's working dive trips are more often than not splitting tips with the captain, crew and any others who may be hauling your gear behind the scenes. If you leave them 3.50$ or 2$ per dive think about what you are actually leaving each person, maybe a dollar.
Yes some of us just make a 'lifestyle choice' but the majority (not talking about the USA or Europe) DO NOT have a choice of just saying 'Hey, this job is not paying so well, I think I'll go to medical school'. Please consider this on your next vacation, when they are clearly making an effort try to leave 5$ a tank. Just add it in when you make your calculations as part of your costs, it's really not a hard thing to do. Your DM will be happier and therefore ideally so should you.
Really, a few dollars means a lot.