Both of the sources you cite list "inexpensive" as a synonym of "cheap", and vice versa.
In fact, I've just looked at the OSX app, and "CHEAP" is the first word after the context in the "inexpensive" entry, and "INEXPENSIVE" is the first word after the context in the "cheap" entry.
http://www.merriam-webster.com/thesaurus/inexpensive http://www.merriam-webster.com/thesaurus/cheap