Unfortunately, the Mini 14 can be classified as an assault weapon, depending on configuration. In truth, an "assault weapon" was a select fire weapon, but the term has been redefined. Now, if a weapon accepts detachable mags and has two other described features it is an "assault weapon." If your Mini has a folding stock and pistol grip it is an assault weapon. Capability to accept a detachable mag plus any two of the following make a rifle "assault": pistol grip below the action, folding or collapsible stock, flash hider, grenade launcher, bayonet lug. A GB is certainly a preban assault weapon by today's definition, but so is my K Mini-14/5F. It has a factory folding stock, pistol grip, and of course, accepts detachable mags.
How is the ATF going to prove that a Mini manufactured before (you pick the date) never had a folding stock with pistol grip on it prior to 1994?
Regarding the value of preban Minis, if a friend of mine wanted to sell a NIB ss factory GB folder for $800 with a factory 30 round mag, it would be his no longer!