30 mill? Wow. Personally, I spend them on the following:
-Max donation for group
-Ninja bond gifts (I like to make my ninjas happy ;P )
-Buying things in Black Market (Even if I don't need the books, for instance, I can exchange them in Space Time for tokens to redeem for things or to bet with)
-Eight Gates (Spam spam spam spam spam spam and hope to get Yellow or Red or at least beef up the Purples I have)
-Spend 'em for that little boost for Nine Tails (10K coins x3 for 30% more damage, I believe)
-Upgrading Equipment (the increase in power becomes incremental and small as you get higher and higher leveled, so at some point it'll not be worth it when comparing the benefits to the price tag)
Otherwise, even the most minor cool stuff is locked behind coupons and ingots. If I had your coinage I'd spend a majority of it on Eight Gates, personally.