It really depends on what someone is willing to pay. However the Light 12's in really good condition made in Belgium in good condition would bring in the neighborhood of $500 +/- $100 from what I've been seeing them go for in the past couple years. If it had a vent rib and was a Magnum, it would be more.
In 2009 I paid $850 for a beautiful 30" Japanese Mag 12, LNIB from 1998. Recently I've seen the same gun go for $1600, never fired in the box