You have to get an FFL (I think it's a Type 07, as long as you're not planning on making armo-piercing ammo). That fee's pretty reasonable - I think it's something like $150 for three years.
You also have to register with ITAR, even if you're not going to be exporting ammo. The fee for that starts at $2250 per year.
There's also a Federal Excise Tax of 11% on all the ammo you make. I *think* the only way to avoid this is if the customer provides the brass, and you reload *those exact same cartridges* and then sell them back to the customer.
Then there's the biggie - insurance. I'll let others chime in on just how much you need, but I'd say that $1 million would be the bare minimum.
You'll also have to deal with a lot of federal, state and local regulations regarding what quantities of components you can have on hand, how they have to be stored, etcetera.
I don't know if commercial reloading can really be done on a small scale - it's always seemed like a "go big or go home" proposition to me.