You typically want to stay clear of feeder fish unless you breed them yourself. Like kno4te said, there's too big of a risk of transferring disease or parasites to your fish. If you want to get some meat in your gt, you could try feeding tilapia, or frozen foods such as blood worms, mysis, krill, etc...
As for pellet food, I typically buy 1kg bags of hikari cichlid gold on amazon for $25. They last my overstocked 210 for ~6 months, so if you just have the single gt, one bag should probably last years.