All Hokages are considered either Super Rares or Premium Event ninjas. None of them are cheap. I would suggest if you want Tobirama, you just put a ton of coupons into Mission Boxes when he is in the prize and try to RNG him.

LOL. You'd have a shot at Tobirama if you got a few hundred boxes. I've gotten ninjas in mission boxes quite a few times when you have like 300 or so. I got lucky for BA 8-9 at 245 attempts out of 300. (1,960 advanced threads just for that 1 level)