Meh.. Good riddance. I never liked Costa much and hated him when it was rumored that he was downing tools when Mourinho was still managing Chelsea. That kind of person has no honor and he doesn't deserve to play in the top clubs of Europe. It's better for him to sell out and do a big money move to China.
You make some good points there to be honest, he is a bit of an asshole. I don't know how he hasn't been sent off more times, always seems to get away with stuff.
Chelsea should cash in, you're right.