You need to be careful when purchasing a Mark V Diving Helmet because there are some very good reproductions out there.  Some have even been purposely damaged and corroded so they look real. There are several good sites on line that tell how to spot a reproduction.  Some excellent looking reproductions can be had for between $100 and $200, but an authentic US Navy Mark V helmets is easily worth ten times as much. One particularly well made fake is made in Korea and is supposed to me a Morse Diving Helmet made in 1941. 
 See the following site.
http://www.geocities.com/giltzowm/html/reproductions.html

I have no problem with reproductions, so long as you know what you are getting. At least one claiming to be authentic on ebay looks suspeciously like the Korean fake to me.