In Northern VA:
Cheapest I've seen was $42 per hour for group lessons, $60 per hour private, $50 per hour semi-private, $47 per hour tri-private. All levels.

More commonly the prices are $50-$65 for group lessons, $70-$90 for private or semi-private. Unless you are willing to drive 90 minutes or more every week. Lots of farms claim to be "convenient to Washington DC" when they're actually  in Berryville or something.