Possibly White Grass Ski Touring Center in the Canaan Valley, West Virginia. I say "possibly" because I haven't been there in 15 years or so, and the snowiness might have changed. Then it had snow when nearby areas did not. As I remember, that was because it was within the lake effect area. We would drive for several hours from the DC suburbs, through totally snowless territory, and then, when we were about to despair, WHAM! Lots of snow on the ground!
I can't remember if it had totally flat areas; the trails I remember were very modestly hilly. An alpine skiier would sneer at their trails. I can't remember how expensive it was, but was very unpretentious, although beautiful. It had a good restaurant attached, but with a very limited menu.
There is also a downhill resort in the Canaan Valley, not far from Whitegrass.