Hi! I could be wrong, but I believe the maximum cost is 1k for 5x buff costing 200 coupons each. The buff is beneficial for groups that need the buff to get the additional coupons for their other members. It also will help for additional group summons in the future. If you already do get the maximum allotted per attempt, then it wouldn't help you as much.

Hope this helps!

~Jib