There are a lot of "zone fare" coupons on Northwest circulating around. Depending on the taxes the fare for your itinerary, traveling in August, would be somewhere around $380-390. In September it would be even less. Check the Coupon Connection forum - people are often looking to trade these or even give them away.

You say you are on Social Security. Does that mean you are over 62? If so, you could buy a senior coupon book for around $700 that entitles you to two roundtrips per year. A good deal in your case only if you plan to make a second trip.