Free: Contests & Raffles.
My Question is how would they know when it was built? You would have to either register it, or have some kind of Proof of Life photo with a newspaper and a date.
Unless I'm mistaken, the new law does nothing to prevent you from making your own firearm unless you're prohibited from having firearms. You can't make one with the intent to sell it, but that's already covered under Federal law. You can also put your own serial number on it at your discretion. Doesn't matter what the number is, and might be a good idea if it ever got stolen.
There used to be a fee that was to be paid to the ATF upon completion of assembly of the parts. I think it's way overlooked among the hobbyists assembling kits.
Firearms (non NFA) made for personal use by unlicensed persons are not required to have a serial number or a unique identifier. If you decide to sell it one day, you are not required to have a serial number or unique identifier marked on the firearm.