There is always a big difference in rates between midweek (Sun-Thurs) and weekend. Also, it depends on whether there is a big convention in town. A couple of nice places that often have reasonable rates are Treasure Island and the Las Vegas Hilton. The Hilton is a couple blocks off the strip but TI is right in the middle of things. Circus Circus and Excalibur also often have low midweek rates, but I personally do not like either of those places.