As other members stated, eBay is your best source for an "up to the minute" price value guide for car kits, in the last 10 years eBay as changed the way vintage collector car kits are priced, before online auction, car modelers had to rely on hobby shops carrying old stock or model car contest vendors to get that out of production kit at astronomical price, inspired by
price guide, rarety & demand of a kit on the market, but with the online auction 1000's of vintage car kits are available every single week growing the offer like never before.
For example buying an original amt '77 chevy monza kit in factory sealed box would have cost approx $50 or more a few years back, I just bought one for $14 a few weeks ago on eBay, but guides states that the value is still $50!
But the opposite is also true, price can skyrock especially if you own a kits with a mold that was unique & /or modified in later years, annual kits like a 67 Johan Deville, a '69 MPC Grand prix , or an original '62 Monogram 1/8 Big T hot rod tub kit will bring top dollars, since the molds of these kits where modified for annual design updating or modified with newer updated parts in the 70's or 80's; they are gone forever in their original form.
Check if your kits are also or have been reissued in the past, this will also affect the value.
A kit that as been out of production for 35 years or had a very limited distribution; Japanese kits for example, an Edai Porsche RSR Turbo 1/8th kit.
Like everything else its a game of demand & offer, but as told with the '77 Monza kit, it doesn't meen that a kit is rare, (this one has not been reissued for over 25 years) that it will bring top dollars, it's more a question of demand &... (emotions & wallet thickness!)