I've never really calculated value for items in the mystery shop, but here goes.

item - coupon value - coupon value per mystery stone

Summer youth Sai clothing - 62 - 1.24

Fukuroku Clothing - 75 - 1.50

Shape Shifting Card - 63 - 1.05

Mood Scroll - 15 - 0.21

Rename Card - 375 - 0.22

Group Rename Card - 1225 - 0.22

Trumpet - 13 - 0.22

Flying Raijin Kunai - 30 - 0.21

Myoboku Gift/Grasp/Training - 10 - 0.21

Purification Rune - 13 - 0.21

Purification Protection Rune - 17 - 0.19

Seeing this each Mystery Stone is worth between 0.19 and 1.5. So, each fragment of Beach Party Clothing is costing between 71 coupons each and 561 coupons each. It seems like they probably set the mystery stone cost for the item based on the 0.22 coupon per stone value that most of the items have which would put the price ~78-82 coupons each. Compared to the other 2 clothing items the price really *s, but don't expect it to change.

Also, don't expect any other items to be added to the store. My guess is that they used the slot for the 2 weeks of genin rank you could buy in that store and they hid a long time ago, to be able to add this item. Would be cool if they rotated different items in that slot each month though.