Based on the article it sounds like Sig is doing everything right. You will be able to order the parts to have a custom-ish P320 all from their website. Not only that, but they are smart enough to include the ability to buy P320 parts/accessories from a variety of manifacturers, not just Sig. This is a model to watch for firearms sales in the 21st century.
$300 MSRP seems reasonable for an FCU to me, especially if that settles down to $200-250. I bet there are plenty of people who will pay a little more for a custom-ish gun.