Common postage dues. I don't have a catalog handy. I know you are new and I am not trying to discourage you from posting questions. Certainly continue to do so.
But there are ways you can also help yourself. The Internet is a powerful search tool. Since
ebay is pretty much the marketplace for this material, queries there will return true market value (provided you have a correct identification of your stamps - that's the tricky part).
For this question you have all the info you need to find something for yourself. If you want to see current listings on
ebay, go here:
http://www.ebay.com/rpp/stampsType "Liechtenstein 1920 postage due" in the search box and see the results. If you want completed listings, click Advanced next to search and check off the "Sold listings" button and see that these sell as a complete set for about $3.
Just one of many techniques for finding info. Another is to go to your local public library, if it is well stocked, and find Scott postage stamp catalogs and look them up.