Mei isn't hard to get in kage. Yes it's a 1/8 chance of getting her but she rarely shows up in events, which make her much harder than getting her in kage.

You need to know how much exactly she is. If you don't have enough coupons/ingots for her this week, you might be just end up with some of her frags and you just have to wait whenever if she might come in an event, which would be a waste of coupons/ingots if you manage to pull her. Ninjas like hanzo appear more often than others like danzo, mei, darui in events. So you don't have to worry about spending to get the full 80 if you don't want to. You need 2400 pts for mei since 80 frags*30 and 2000 for hanzo since 80 frags*25. If on average, a dice that costs 20 coupons/ingots and you get 5 pts per roll. Keep in mind you can get 10 pts by hitting a certain place on a board.