He very clearly is being sarcastic, because he knows the 20 million claim is bullshit. He's basically friendly teasing them because if it really WERE that high,

He very clearly is being sarcastic, because he knows the 20 million claim is bullshit. He's basically friendly teasing them because if it really WERE that high, then buying Apollo for 10 million might be a good idea, but it isn't and it's all bullshit, so he knew they wouldn't. That's why he says "It's mostly just a joke"