Kaku [OP07-080]

I have "OP04-089 Bartolomeo", a card with a base cost of 3, on my field, which currently has a cost of 4 due to this Leader's effect. During my opponent's turn, my opponent uses the [On Play] effect of "OP07-080 Kaku" to give Bartolomeo −3 cost. What would Bartolomeo's cost become in this case?

In this case, Bartolomeo's cost would become 0. Usopp's effect is a permanent effect, meaning it is processed first: this raises Bartolomeo's cost from 3 to 4. Kaku's [On Play] effect, which is an auto effect, is processed after that, dropping Bartolomeo's cost from 4 to 1. As this means that Bartolomeo's cost is now 1 or less, Usopp's effect would no longer apply to it; it would lose the +1 cost from Usopp's effect, so its cost would become 0.