One obvious cost is financial - you come to church and are expected to give of what God has entrusted to your care. And not just a dollar or two. God's people are expected to tithe - ten percent. That is God's standard, not the church's. And on top of that are all the other worthy causes which deserve our help.
Discipleship can get expensive.
You are expected to give time. Jesus talked about visiting the sick, feeding the hungry, clothing the naked, and doing it as if we were doing it for him. That takes time, often time we might rather be spending in other pursuits. Our talents are on call as well. Then your Sunday School needs you and ought to have you.