If you want to gain a basic idea of how much an artwork is worth in the art market, your best bet is to find a record of the sale of another art work that is as similar as possible (same artist, same medium, same size, same date of creation, recent date of sale or as many of the above as you can). Use the links and resources listed on this page to look up art sales.
That being said, appraisal is a complicated topic and there are experts who have studied it and do it for a living. If you're interested in contacting one of them, try one of the following:
