I just priced a Vegas vacation for January 20-22 for 2 people including airfare, hotel and rental car at a total cost of $338. The airline is Southwest and the hotel is Four Queens. The rental car is through Payless.
Thats a pretty cheap trip. Of course you'd also have to pay for food, entertainment and transportation when you're there. You can find a lot of cheap food and entertainment in Vegas.
Airfare : 2 tickets on Southwest for $300 total with 50% off sale
I discussed earlier that you can get 50% off on flights to Vegas via Southwest airlines. Airfare rates will vary depending on where you are flying from but I did a few checks and it seems that you can get flights to Vegas from most US cities for around $100-200 range.
Hotel : 3 nights at Four Queens for $11 via Expedia with 'fan50' coupon
Theres also a deal where you can get $50 off 3 night hotel stays from Expedia by using the coupon code 'fan50'. If you search Expedia for a mid week stay in January you can find pretty cheap deals especially for the downtown hotels. One dirt cheap option is at the Four Queens. They have a promotion it seems where the 3rd night is free and if you combine that with the $50 off for 3 nights the rate is as low as $10.96 for 3 nights.
You can shop around and find deals for other hotels on Expedia. Three nights at Circus Circus ran $42.13 or the Sahara was $47.85. These are all supposed to be 3 star level hotels according to Expedia. I'd say they are closer to 2-3 star level. One thing to consider if staying at Vegas in the downtown hotels is that they are farther away from most of the attractions and sites on the popular Vegas strip. So spending a bit more to stay on a strip hotel could be worth the cost especially if you're using taxi's for travel.
Rental Car: 3 days economy car for $27.41 from Payless via travel.yahoo.com
I didn't have any special discounts for rental cars. I don't know if Payless is a good company or not. Enterprise, Alamo, Dollar and Thrifty are all in the $28-34 range so you have a number of reasonably priced options. Rental cars are dirt cheap in Vegas in the winter so this would be the most practical transportation option. By comparison, taking a taxi from the airport to the hotel and back would run at least $20-40.
Total cost: Airfare $300 + Hotel $11 + rental car $27 = $338
Your price may vary depending on exact airfare. But I figure for most cities in continental US you can get a midweek winter trip in Vegas for 2 for around $250-450.
Reference: Fatwallet post on the $11 hotel stay
Photo from Wikipedia