Since the event is in Fairfax County, a general rule of thumb is to check the latest valuation of the house of the celebrator and divide by 2000 for an appropriate monetary gift. This will give you an appropriate gift whether they live in Great Falls or Chantilly or anywhere in between.
You can check the valuation here
http://icare.fairfaxcounty.gov/Search/GenericSearch.aspx?mode=ADDRESS
Shallom and Lachiem!!!