Check out this on-line auction site that has a Class III Section. This will give you some idea what guns are going for in the free market. http://www.gunsamerica.com/
Keep in mind that if you saw something you liked, you would have to find a Class III dealer willing to do the transfrer and pay him whatever he decides to charge you, on top of the price of the gun. Then you have to file your application with BATF and pay them the $200 transfer tax. Please note that the price of Class III weapons has skyrocketed in the last few years. This is the result of the Clinton administration (thank you, anyone who voted democrat) and the assult weapons bill. This legislation stopped the manufacture or importation of new guns. So, since the ones out there are the only ones that will ever be out there, the price went through the roof. I can remember when you could buy a MAC 10 for $350 plus the $200 transfer tax. I remember when a local store had an M-60 sale going on for $1800/each. It is also interesting to note that prior to the National Firearms act of 1934, you could buy machineguns at the local hardware store. You could go up to your local store and buy a 1928 Thompson and there was nothing out of the ordinary about it. You could buy a suppressor so that you would not disturb others or hurt your hearing when you shot. But, I guess that crime in 1934 was so bad they had to protect society from these evil menaces, and we are still paying for it today. The NFA of 1934 is what extablished the $200 transfer tax. Keep in mind that at that time, you could buy a new car for under $1000, so this tax was designed to prevent private ownership of machineguns.
This is just a guess, but I would say that owning Class III weapons in the Chicago area would be pretty much out of the question. That area was lost to the democrats long ago. If you come to the Southwest, in the states that are still free, Class III stuff is pretty common. We have full auto matches at the shooting club I belong to. I was shooting in the desert the other day and there was a guy shooting his MG34 not far from me. We have several indoor ranges in town where you can rent full auto weapons. Basically buying a full auto weapon involves a couple things. You have to file an application with BATF and included a transfer tax of $200 for the weapon. The application has to be signed by the head LEO in the area you live. For me, that would be the county sheriff. Now obviously you have to live somewhere that has no local laws against class III weapons (or any weapons for that matter) and the sheriff (or whomever) has to be willing to sign the application stating that there are no local laws prohibiting you from owning this weapon. The turn around on this application may take six months during which time you cannot have the weapon. Instead of reading a bunch of speculation and heresay, go to the link already listed and read all this right from the BATF web site. This is not a "license" to buy a Class III weapon, this is just a transfer tax and a background check. You have to go through this proceedure for each gun you buy and if you want to sell one, the new buyer will have to go through this before you can sell. This transfer tax is what caused the BATF/FBI to murder the entire family including the dog of Randy Weaver. The news media made a big deal out of him selling a sawed off shotgun. Sawed off shotguns are perfectly legal in this country as long as you jump through the BATF hoops and pay the transfer tax.