Goku was confused because Vegeta decreased his power for no apparent reason.He says it will cost power but doesn’t specify. I think the fact he even brought it up makes it more significant than a 10% loss. Specially when Goku is confused at Vegeta sacrificing his power and the fact that’s the first time Vegeta is said to be weakened. Not that the blast didn’t weaken him, but the ball was the real loss.