I did a quick search on TISS.com for outbound 4/9 and return 4/12 (it wouldn't display any fares for a return on 4/11) from Washington area airports and it gave the following:

Icelandair 270.00 USD
Continental Airlines 369.00 USD
Major European Airline 504.00 USD

Unfortunately (I'm guessing because of the short notice) you need to call them up to check availability for any of these fares.
Their contact information is below.

tiss Ticket Center America
(East Coast)
96 Engle Street
Englewood
NJ 07631, USA
Phone: +1-(0)201-541-3826
Fax: +1-(0)201-541-3829
e-mail: [email protected]

Even though I'm assuming your mom isn't a student, Council Travel are also offering a $259 r/t fare to non-students on Icelandair on the date you wanted -their number is 1-800-2 COUNCIL.

Good luck!
Yonatan