"...Do you think reloading will give me a significant advantage over $85 for a box of 20 300 win mag?..."

I think that depends on how much you shoot. If you only touch off 15-20 rounds a year, then no, you would be better off buying factory ammo.

But if you shoot a lot (and keep in mind that you can reload reduced rounds that won't beat your shoulder to death), then reloading will pay for itself in a reasonably short time. After that you will be in the green all the way...