See the following site for a water activity meter that might work for you.  <http://www.aqualab.com/products/water-activity-meters/pawkit/>.  They do not list the price but as I remember it is several hundred dollars.  

The other option is to buy a relative humidity meter that has a probe you can insert into the lid of a jar.  Put the prunes in the jar, close the lid, and insert the humidity sensor.  The relative humidity (RH) should be fairly constant in a few minutes.  Divide the RH reading by 100 and you have water activity (Aw).  One caution, all electronic RH meters need to be checked for calibration.  A calibration kit costs about $50 or so. 

                                                      --Jim Thompson