Are you talking about the machinegun ban of 1986 or the actual NFA '34? If you are referring to the NFA of 1934 then there was no difference between the pre and post machinegun prices. The only difference was the addition of a $200 tax. Up until 1986 most machineguns were about the same in price as their semiauto counterparts, like M16's and AR15's for example. Others had risen in price since they could no longer be imported since the 1968 amnesty, such as original H&K MP5's, which is why you see so many sear guns and converted semiauto receivers out there and no original factory weapons.
I've got several old Soldier of Fortune magazines stashed somewhere that had a few prices for machineguns. I was looking at them about a year ago wishing I could only go back in time.