do stores sell pre cooked chicken breasts that have been grilled, and are boneless? does anyone know if they're more expensive, or if theres any nutritional downside to them?

Some grocery delis do and they're always more expensive. You can get prepackaged stuff, too, but they've usually got some coloring, preservatives and other things in them. These, too, will cost you more. I grab 'em at Costco and bake up a batch on the weekend.

Pre-cooked is always more expensive. Personally, I buy the boneless breasts and cook a batch every few days. Then just leave them in the fridge (sealed obviously) and zap them in the microwave at mealtimes