They didn't technically make a 1964 Mustang
. The first year was 1964 1/2 - Ford often introduced models or changes to models and called them "1/2" year models.
Since this was the first year for the Mustang, it could be worth a little more than salvaged later models. It really could depend on the condition and originality and options. An original V8 Mustang convert could be worth several thousand dollars if it's complete with original motor, transmission, etc.. but just in non-running condition and needing everything.
If it doesn't have the original drive train, needs most all body panels replaced, has rusted through floorboards, etc... but is complete with VIN and title, it may be in the $1000 range - less if it has no interior, motor, trans, rear, top, etc...