When LE/MIL buys in quantity, the bid cost on the contract is under $700, fully equipped, milspec. That is directly from the manufacturer shipped to the receiving inspection point with all documentation. And, full auto when required.
We don't get that. Our rifle would be shipped to a distributors warehouse, palletized in the boxes. That pallet gets broken down and reshipped 10 or 20 rifles to a large vendors warehouse, to be further distributed to an actual retail seller. The gun gets marked up at at least twice to cover the intermediate and final shipping and handling. The retailer has his price point to cover his overhead and make a profit.
Makers markup + distributor's markup + retailer's markup = selling price. You may find some cheaper, or even discounted when the public isn't buying quickly. You may find that the contract bid price could even be as low as $450, but the makers get concerned when sales are slow, too. Overall, we could be buying Norinco's which sell in Canada for CN$550, which is close to US$400. But those aren't allowed here now by trade treaty.
Which leaves us with the 100% US made and sourced versions. And since we won't work for less than $10 an hour, we can't expect to get $400 US made rifles. More like $1000, which we only have ourselves to blame with the cost of living we insist on and the multiple steps in the distribution chain that are now institutionalized.
Or, buy a S&W for $650. Which brings up the point, how much of what we are getting is based on reputation rather than actual superiority in manufacture? Does a Colt shoot $400 more accurately, or a deer shot with one $400 more dead? No, that's all about the ammo, not the launching tube, and it is very arguable there is a significant difference. What difference does exist is largely the amount of testing done to ensure the parts meet specifications - and that documentation is never provided to civilian buyers. Only per government contract.
At best, we buy the seller and move on.