The cheapest time to go would be the summer, July and August because the temperature is over 100 degrees and they have to lure people into the heat with lower room prices. It isn't really that bad, though, you get used to the heat very quickly because of the lack of humidity, and it is great for hanging out at the pool.Room rates right now are very low because of the recession, I'm not sure what is going to change next year. As a rule, each hotel has a normal sale rate that they offer all the time. Those rates can be found here
For cheaper hotels,
Excalibur is located on the strip, is a decent hotel with a great casino, 4 pools, nice widescreen rooms, and close to the action. They have rates right now from $22/night. Before the recession their sale rates were around $50/night.
Luxor, which is a pyramid and is next door to Excalibur, is a little more upscale than Excalibur but they are still running rates from $39/night. Before the recession it was $65/night.
Harrah's usually runs around $55/night on sale and is in the center of everything.
You will find that Stratosphere, Sahara, and Riviera are the cheaper hotels on the strip, but that is because they are located nowhere near the other hotels on the strip. You would have to take the bus to get anywhere.
Keep in mind that any hotel room you book, the price is based on 2 people staying in the room. With 10 people total, you could rent 3 rooms and put 3 girls in 2 of them and 4 girls in one, but if you tell the hotel that you are sleeping more than 2 people, they will charge you around $30 extra per night per extra person.
For example, say you find a room rate of $40/night. You need 3 rooms which is $120. Well that $120 only covers 2 girls in each room (6 girls total) so the hotel would charge you $30/night for the other 4 girls totaling another $120. What most people do is only put 2 people on the reservation and then kind of sneak the others into the room.
Good Luck!