Rather than a buyout clause, I wouldn't recommend signing any non-exclusive contract that does not allow cancellation, with notice, at any time. A non-exclusive should, by nature of what it is, be relatively non binding. Also, if it is non-exclusive, there wouldn't seem to be a need to buy any contracts out since a non-exclusive provides the talent opportunity to work with a variety of agents, hence the term itself as being 'non-exclusive.'
To contribute, here's a few articles I have written on this topic:
Signing With an Agency
http://www.onetalentsource.com/view_article/4/
Agency Contracts: Exclusve vs. Non-Exclusive
http://www.onetalentsource.com/view_article/21/