Depending on which kit you have Monogram vs Revell Ag, the kits were only about $20-$30 when they were originally released. The Revell Ag kit came with better decals (I think they were from Thomas or PNT models) and it included a collectable pin.
Quite a few years ago Monogram and Revell merged and combined under the Revell name, although they moved from their HQ in Venice, CA to Monogram HQ in Chicago, IL.
The price you see is a reflection of the popularity of the kit combined with it's availability. Another example of this is original Battlestar Galactica, while it is a terrible kit that again sold for about $20 during a rerelease, now sells for about $100.