Honestly, hotels are usually known to have more expensive parking, because their guests are largely a captive market. Not all DC hotels have their own parking, and those that do usually don't have enough parking for every room (knowing that many guests will arrive via Metro or Uber).
If you want cheaper parking, watch Spot Hero and other parking reservation sites. More parking will become available as we get closer to the con, for some reason a lot of garages don't like to reserve overnight parking 5-6 months out. In general though, I'd expect $25-$30 per night.