The gap from 50 to 70 is 2136 EP, or 43 stones.
I don't know a lot of people who max out a ****+ every week (especially on top of hitting 35 with a non-plus). Those who do are buying a lot of chests, most of which ends up as enhancement fodder. Of course, fusion stones are also a reward from gold and silver chests, which come out of epic boss awards, so hoarding rates may vary.