The cross over event is using the old exchange rate shop as we were suppose to have this event a ways back, that or because it's a month long. Regardless, this means you can only buy 3 sets of 10 for 300 soul p, which isn't even enough for you and more costly each set. The upcoming Horus event (coming out tonight) will likely use the current cheaper rates and have 600 soul p available, so you're better off just farming the Horus event ones.