It's been about a year since I have loaded any shotgun shells. I don't know about now, but then components were spotty, and shot when you could find it, was pricey. Surprisingly primers were always available. I got mine from Bruno's in a 1500 count package, that were European made.
There was a site that calculated the cost of a box of shells when you entered the cost of your components in. I'm sure Google can help you with that. The Federal 100 pack from Walmart might be your best bet. A lot of guys didn't like to use Federal shells, but as someone once told me, if you have your barrel pointed in the right spot when you pull the trigger, it really doesn't matter which shell you are using.