Hi, I'll give my thoughts on this although my experience is focused on the traditional / waterfall style of projects.
I agree that agile teams would involve more collaboration with the customer, however, I don't think you can say the contract value would be lower necessarily. The idea of open collaboration with the customer would help the team to stay focused on their needs and get their buy-in regularly. However, more meetings with the client would result in more hours being spent by the contractor. The work does not change, but how often you're communicating / interacting with the client does.
I'd be interested to hear others' opinions.