No machine guns manufactured after 1986 can be legally owned by a civillian. Part of the firearm owners protection act. Previous laws in the mid sixties and the aformentioned laws put in place in 1934 had already diminished the overall count of legally transferable machine guns by a huge margin. At this point, because of supply and demand, a fully transferable machine gun can cost anywhere between 10k and 80k, depending on a whole host of facors. Normally mabufacturer, model, and condition.

And you are right, making a machine gun is cheap. Converting an existing rifle is even easier. With wire from a coat hanger, the internet, and twenty minutes anyone can make an ar15 fully automatic. The thing is, no one does that, because the penalties are severe and despite what the media shows on tv, most gun crime in the united states is carried out by small caliber pistols. But thats a conversation for another thread.

Fun fact, if you want to legally own a rpg, and can pass the background checks, its all yours. Its just ungodly expensive and it also requires a 200 dollar tax stamp. And each projectile requires the same background check and individual tax stamp. I imagine you can guess how hard those are to come by, and how much attention your address would get after trying that.