Actually, that is a special limited thingie in the gacha corner. I imagine this thing they are implementing on nutaku is the very same deal. It appears now and then for limited times, the other time being a step-up gacha.
The 5k thingie is meant for those that REALLY REALLY want a girl of their choice, not us standard f2p players or casual payers that pay 1k nut gold a month at most.