Jan is convention season as others have pointed out. Huge difference in price from December to January. That being said, still good deals to be found.
http://deals.lasvegasadvisor.com/hotels/
Can also use the app Hotels Tonight. Wife and I have scored newer strip hotels on a Fri/Sat night for under $50 if we want to get away from the kids for an evening. Plus if you're new to the site they give you $25 to try it out.