Converting an AK47 requires machining the receiver. Just machining the receiver will get you 10 years in Federal prison even if you don't install the autosear
There were less than 100 bring back AK47s registered before the 1968 ban on imported machine guns. They start at around $25,000. There were about a thousand Egyptian and Chinese semi-autos converted before the registry was closed in 1986. One of those will run you about $13,500.