Midway's prices for lead are astronomical and are not good for calculating price comparisons.
Most folks use lead wheel weights that they get for a pretty low price (I picked up 200+ lbs from a tire shop for $50 bucks not long ago). The advantage to casting your own is that you get to select the exact style of bullet that you want, you choose how hard or soft the bullet is, you choose the bullet lube, and you can size the bullet to fit your gun.
It boils down to having complete control in regards to what you feed your gun. That's why I cast bullets.
For a wealth of knowledge about cast bullets, check out this site: http://castboolits.gunloads.com/index.php