Full-auto guns don't have to be expensive. Any guy with a machine shop could build a full-auto gun for very little money. The M3A1 was ridiculously inexpensive and easily manufactured. A shop that was set up to make them could probably crank them out for under $100.00. An M16 should cost very little over the cost of a standard AR15. The Full Auto control group and a little machining is all that is required.
Ask the Iraq or Afghan vets how many full-auto AKs they saw in the hands of the citizens over there. Those things are really easy to get. The Soviets and Chicoms made millions of them.
The problem is the law in the US. They don't want us to have full-auto weapons, so the licensing fees are expensive, the regulations are byzantine, and the consequences of not fulfilling even one little jot or tittle of the regulations means loss of your business, exorbitant fines, and jail time. It just isn't worth it.