I have read that Bruno Mars and most performers who have played the Super Bowl did so for free. The exposure these performers get is supposedly so valuable it is worth it to them. I am in a band that has been booked at a fairly large festival in my area (10K plus people with multiple stages). We are not getting paid other than extra tickets and food/beer. The gigs we have gotten since we have been added to that lineup have been better/more lucrative AND more people are checking us out online and coming to gigs. While playing for nothing is usually a bad idea, it is not always so.