I have worked with some of the Top 50 Fortune 50 companies for Mobile/Web/Google Glass Development Projects.

Never work for free if you don't want to devalue yourself later. If the client is big (read rich) he won't mind paying for some initial Mockups.
The whole point of working with big clients is so you can charge them more than you would charge an average client.

Let me know if you wish to talk about it. Willing to talk for free :)