Same here, i have the tri-fecta: carbine (Marlin Camp 9), carry piece (Kel-tec P11), range toy (S&W 5904) that all use the same mags. Makes a nice 'bug-out/shtf' group (not that we talk about that sorta stuff here).
I bought mine long ago for alot less, but have seen them go on several auction sites in the $600+ range recently. I suppose because they're becoming rare?
$400 in good condition seems about right to me. Nice acquisition, and enjoy it!
Also, a suggestion to check (and probably replace) the buffer before you start shooting it. They have a tendency to get brittle with age and crack into a million pieces. Purchase them in two's.