For the cheapest Beiben 6x4, 400hp (or similar) tractor trucks, look at online heavy machinery marketplaces like Autoline (from ~$15k) and Made-in-China.com (from ~$21k for new, ~$8k for used Sinotruk), focusing on used/second-hand models and specific sellers for the lowest prices, but remember "military" usually means rugged civilian specs, so check sites like Alibaba or Beiben Trucks for direct factory/dealer deals on specific models like the 2642 (420hp) or 2638 (380hp).