I'm looking at our pricing, which is well under MSRP, and neither Da-Lite, nor Draper, offers any screens which are available from them directly for under $400 at cost. The least expensive would likely be the Da-Lite Model C which runs $804 to $1,004 MSRP at 159" diagonal, or the Draper Luma 2 161" Matte White which runs $788 MSRP.
Either of those might be in the $500-$600 range drop shipped to your door.
The big issues you run into are that screens of that size have a fair bit of weight to them which requires heavy duty rollers. At the end of the day it is a lot of material, a heavy screen, heavy duty parts, a heavy duty case, and a lot more weight that must be shipped.
Otherwise... ebay is your friend...
http://cgi.ebay.com/Elitech-150-MANUAL-PROJECTOR-SCREEN-16-9_W0QQitemZ180307305482QQihZ008QQcategoryZ25323QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
