viewing a 1:24 car from a foot away, it will be as if seeing the full size vehicle from 24 feet away.
the monte carlo in the bush series was about 200 inches long (give or take) divide that by the length of your cars and that will get you close to the scale (depending on the maker of the car).
the scale of your cars should be on the tin and the cars, along with the makers name.(action, winners circle, hot wheels, etc.)
1/18 will be around 11-12 inches long...at about 200.00 each
1/24 7-9" 70-100.00 each
when i get home monday i 'll try to take some pics. of some of my 1/24
maybe that will help
here is a pic of some scale cars that i found online.