"Classic cars" is a term that usually refers to a nearly original older cars. You can use Google to find "Old Cars Prices" or "values", or "automobile auction sale prices". This will give you an idea what they are currently selling for, depending on their condition.

If you mean hot rods or customs, it is harder to determine a market value. It's also harder to get your money back in this area, as it usually costs more to build a car than it is worth. In this case, make sure the car is one you really want, and get some of the value back in enjoyment, and finding new friends who love cars like you. Buying a car that is finished, or nearly finished, is the best way to save money.