You should be absolutely thrilled that someone gave you money for barnyard mix bantams. That's absolutely amazing to me.
I have to agree with 20736--there isn't a market for roosters unless they are purebred, and even then most of them are unsaleable. People do not want trios of mixed breeds--trios are for show birds. I think if you require people to buy trios, you'll lose your hen customers.
Look for a livestock auction near you and accept that you'll take a loss on the cockerels, or feed them a higher protein feed and butcher them, or offer them free on Craig's List. We do all three of these.
I also agree that your hens should be priced higher in order to make up the loss on the roosters. And have you considered selling chicks straight run instead of started birds? That's what we do. Then all the babies are sold regardless of gender. Yes, you only get a few bucks per chick, but you don't have much feed in them and your costs are very low, so almost anything you get is profit.