You can get him to accept for a much lower amount. The persuasion bar won't fill up, but you can still propose a much smaller amount and he will accept. Lords usually go for 30k-100k, but they will all ask for ~1m if you click the "automatic deal" button.
EDIT: Some will also join for free. The bug seems to be with the green bar not correctly representing the lord's willingness to accept in this case. The actual barter cost is balanced.