I would check with local drug dealers and other shady/questionable characters. They might be able to hook you up with something stolen for that price.
Otherwise the word cheap does not belong in the title here....honestly I don't think there is a standard HU that you can hook up a external PC DVD player too....you might try a Alpine Entertainment unit...
Else you might check out the other builds people have done on this site and check out some fold out screen from digitalww.com
Hope that helps at least a little bit.