Red Dawn is a cult classic, it's success is not measured in box office receipts.
A lot of movies in the 80s stuck.
Red Dawn was a great movie. Unfortunately, you had to know a little history and understand the implications of current events in order to realize how good it was. It was trashed before it hit the screens as "red-baiting" tripe by the usual suspects, though it was nothing of the sort.
The movie suffered from a well organized effort to convince people not to see it. The same thing happened to "The Beast", a very well-made, well-acted and authentic movie about the Soviets in Afghanistan. They were both anti-Soviet, and in the 80's, the left couldn't allow that, especially at the movies.
Red Dawn grossed nearly $36M in the US:
http://us.imdb.com/Business?0087985
I wasn't able to find an authorative reference to the cost of production, but several reports put it at 8.5M. From what I remember, it wasn't exactly a high-cost production.
That's hardly a flop.