My 2 cents is that I typically only run email submits as an add-on to a campaign that's already profitable or breaking even. Adding the email submit adds some incremental upside that often helps me reach my 50% margin threshold for my campaigns.
Let me give you a made up example...
If one of my primary PPC campaigns is for WeightWatchers, because it converts at break even or above - what I would also do then to take it up a notch is somehow get my secondary offer, maybe a Weight Watchers email submit (or another relevant weight loss offer) into my offer funnel (I do this using a variety of methods) .
I have personally have examples of this where my secondary offer covers all of my advertising expenses and my primary offer is all profit.
I am also a member of Gauher Chaudrys PPC & PPV Formula forums. Gauher and many members swear that a lot of money is still being made with pure email submits... but that's not my experience or expertise.
One last thing I do is to rotate my email submits between 2 and 3 similar offers. I find that this helps keep the volumes lower (of course) per offer and it seems to reduce attention to my leads and related scrubbing. I used to test for the best offer and then just run that one, but I've had 30 & 40% conversion rates that quickly went down to 5-10%. So I run similar offers that may not all convert as well initially, but which deliver me a better epc in the long run due to less scrubbing.
Hope that helps.
Cheers,
Peter