How much is a 1 cent stamp with George Washington on it?

You would have to be more specific because there are many 1c Washington stamps. Look up the stamp you want to find out about in Scott's Postage Stamp Catalog (usually available at your local library) but be forewarned there is often a lot of studing and testing that needs to be done to determine the difference between early US stamps.
The US has issued many stamps with George Washington of many different values, including a number of 1 cent values. And if it is unused, you can always use it for 1 cent postage. You will need to consult the catalogs and do some research. There are lots of stamps issued and they vary in value from almost nothing to some nice values.

Most libraries will have a copy of Scott's Postage Stamp Catalog of the World. First of all, you have to identify the specific stamp or card. That in itself can be a daunting task, but most of the catalogs are well illustrated with lots of information. Once you know what item you have, the catalog will provide a set of prices that can be used to estimate value.
George Washington is one of the most common subjects on US stamps. It will take some detective work to narrow down the search. Use a catalog online or through your local library to identify the specific item. It will also provide an estimated value.