For cheap Achievement whores red box has games now of course. Rent Bodycount for an easy 600+ points for $1.50. You can probably get 900+ if you keep it a few days but I'm playing poker tomorrow night so I'll take the easy 600-700 and drop it off before I play.
Not a very good game but fun to keep the headshot streak going. The equivalent of spending a quality 6 quarters in an arcade game.