I am looking for video games for my new 360 and would like to get them at the lowest price. Games like - Call of Duty: Modern Warfare 2 - Halo 3: ODST - Left 4 Dead 2 Any help would be greatly appreciated

Well the absolute best deals with probably be on Ebay! Don't let the used games turn you off, I find that most people who have "used" games selling on Ebay, either beat the game, or they just got tired of playing it! Now don't get me wrong, you will not get a fairly new or particularly popular game used or new for 5 bucks, but by going with used, you will save a good $10.00 to $15.00 a game!
For Call of Duty, you can try an auction where your bids start at as little as $1.54, or you can buy it now for $38.99
specifics are:
Return to today's war front with the second chapter in the Modern Warfare series. Modern Warfare 2 continues the gripping and heart-racing action as players face off against a new threat dedicated to bringing the world to the brink of collapse. Playing as as Sgt. Gary "Roach" Sanderson, your international squad of elite soldiers battle through harrowing combat scenarios that range from snowy mountaintops in Siberia to the crowded streets of Rio de Janiero, Brazil and beyond, facing a terrorist force hellbent on redefining the boundary lines of today's modern warfare. Players can face the threat alone or with other players in cooperative SpecOps mode. For even more intense combat, take up arms against online foes in enhanced deathmatch warfare on a host of Modern Warfare 2 maps.

As for Halo ODST you can bid on an auction for a starting bid of $.99cents, or use a buy it now auction for $29.75!
specifics are:
Halo 3: ODST's tale focuses on the legendary ODSTs or "Orbital Drop Shock Troopers" as they drop into the ruined city of New Mombasa, looking for clues behind the Covenant's catastrophic attack on the city.The new campaign chapter predates the events that unfold in "Halo 3" and gives players a new perspective through the eyes of a new hero in the "Halo" universe. New multiplayer maps ensure that the award-winning Xbox LIVE experience of "Halo 3" continues to thrive.

With Left 4 Dead 2, you can put a bid in for as little as $1.29 or use a buy it now for $32.75!
specifics are:
Down in the bayou of Louisiana, all hell has broken loose in this second entry of the multiplayer zombie-fest. Players assume the role of four new survivors, each with their own personality and new dialog. The game leads them through the southeastern region of the US -- from Savannah, Georgia through the bayou country, and climaxing in New Orleans' French Quarter.This co-operative action horror FPS takes you and your friends through the cities, swamps and cemeteries of the Deep South, from Savannah to New Orleans across five expansive campaigns. You'll play as one of four new survivors armed with a wide and devastating array of classic and upgraded weapons. In addition to firearms, you'll also get a chance to take out some aggression on infected with a variety of carnage-creating melee weapons, from chainsaws to axes and even the deadly frying pan. You'll be putting these weapons to the test against (or playing as in Versus) three horrific and formidable new Special Infected. You'll also encounter five new "uncommon" common infected, including the terrifying Mudmen

Go to the Movie trading Company.

A seller on http://www.eBay.com has a Call of Duty 2 available for $10.95 with FREE shipping, the item ID number is 120530362703. There is another seller that has Halo 3 available for a starting bid of $14.99 or a Buy it Now of $18.99 plus $3.99 shipping; the item number is 230437758550.

Hi there.  You have come to the right place to find the XBox 360 games.  We can find you the lowest prices also.  Now lets start with your list of games. 

Call of Duty: Modern Warfare 2:
After a long line of games, Modern Warfare 2 has dropped the historic focus.  It has created new characters.  It is now on new ground by taking the first-person shooter genre to new locales.

Halo 3: ODST:
Another first-person shooter game.  This is an expansion that no Halo game fan should be without.

Left 4 Dead 2:
Has an unusually fast turnaround time.  It is a much better game than the first. 
Please take a look at the following pictures and prices of the games.  I hope I have been able to answer all your questions and find the information you were looking for.

 

If you don't like Ebay - as most people don't. Then you can find Call of Duty Modern Warfare 2 right now at Toys R Us for $39.99

Barnes & Noble currently has Left 4 Dead 2 on sale for $39.99

and for Halo 3: ODST deals, you'll want to check out your local Costco if you have one in your area, as they put this as low as 30 dollars.