The cars should come in 2 size. 1:68, which is about the size of a Hot Wheels car, and 1:24, which is about 6-8 inches long.
1:68 cars generally run in the 9.99-16.99 range, 1:24 in the $25-$50 range. Some cars are more expensive, but they are generally rare cars that the initial run has sold out on. (ex. I recently bought a limited edition 1:24 Dale Jr 2001 MLB Allstar Car on E-bay --- $170 -- and that was a GREAT price for that car. I have seen them as high as $600)