A freeplay is worth around 65% of the notional value.
Because you can do this with it:
http://www.sportsbookreview.com/bett...ue-calculator/
If you pick at 50% then you will lose around 5% of your turnover. So just to keep it simple, say you would expect to lose about 50% of your deposit after rolling it over 10 times.
So if 65% of the freeplay is more than 50% of the deposit amount, it could be seen as a +EV deal in those circumstances.
But that all goes out the window if you don't size your bets to match the deposit amount. Which is what a book would rely on us not doing to bring that sort of offer back to being +EV for them.