I think what I posted before was that it took me about 60 hours to complete the interning requirement for my DM. I guess you can propose that anything beyond 60 hours is your "gift" to them.
So the deal to them could be calculated on the minimum wage of, what is it, $5.35 or so. Say that you would intern every OW classes for them beyond the 3 for free, for another 6 month, 9 month, or a year or so, depending on how many they do a month? Free air fill? Half price on all certs and specialty class you assist with and complete?
Divide $600 into the 5.35, and you get like about 120 of free labor you own them. Add it to 60, and that makes it 180 hours. Divide it into the hours per CW and OW class, and it will be approximately 10 complete OW class you'll need to intern for free as an assistant.
You'll gain great experience, they'll have another hand to assist in the pool, air is free, cost of your specialty courses are half price? It is a win win situation right?
But you'll have to prove that you are an asset and not a liability. I would suggest always carrying extra mask, snorkels, orings, regs, weights, and gears with you. And they will appreciate your help even more. My guess is if you sputter as a DM intern, the relationship will end very quickly. Being efficient in getting your gears on and off, helping students into and out of the water, and simply being a good dive buddy will serve you well.