Marriott Perimeter Center, Fairfield Inn at the Perimeter or Crowne Plaza Ravinia are all nice. If you search on hotels near Dunwoody Marta, you'll be able to find them and others.
Anything within walking distance of Lenox Mall. There's a Marta station there.
The other train stations on the north lines are Lindberg, Brookhaven, Chamblee, Doraville, Dunwoody and North Springs. If you do a search on any of those and nearby hotels, you should be able to find something to your liking and in your price range.