Besides being an old early version of the AR-15, the Sp1 is a version that had the ability to swap out full auto internals like the sear mechanism and easily(no machining), convert it to a Class3 machine gun or true Assault Rifle.
Since real M-16s go for around $16,0000 you can see what a conversion from AR-15 to M-16 would go for. I have an Sp1 myself, I was told average price would fall between $1,200 and $2,000 before this new round of craziness got going. Today, $3,000 would be pretty easy to get I would think.
I have been considering the same thing, sell mine and buy a new one later if/when the prices go back down. I don't even want a machine-gun.
Colt M1911, AR-15 | S&W Model 19, Model 27| SIG P238 | Berreta 85B Cheetah | Ruger Blackhawk .357MAG, Bearcat "Shopkeeper" .22LR| Remington Marine Magnum SP 12GA., Model 700 SPS .223