it's actually lower, i tried it once to go up to the max roll because i wanted the skillbook of gaara that was there (i was lucky and i got it at the 7th, but i was still able to see how much costed to roll the last time) and i can tell you that, unless they changed it lately, the roll 1 is 100 coupons, roll 2 and 3 are 300, roll 4 and 5 are 500, roll 6 is 600, roll 7 is 800, roll 8 is 1000.

The total is 4100.

@OP: is worth to go for them only if you are a super whale and you plan to get all the 7 of them to increase a level 10 refined piece of gear with at least a 97% gold initiative on it, and only if you plan to do it for 3 times, so that you are able to hit the cap of the +20%.