That's tough. If you go on Newegg and just pick the filter "Dedicated Graphics" (which is what you need for it to be any sort of gaming machine), and look at the lowest priced laptop
it is right at $600. I got mine
for $550 but it was a Black Friday sale, so you can try shopping around for a sale. You can get something like this
, which has a pretty decent graphics card if you're willing to spend a little extra. That one is almost the equivalent of mine (I can play L4D at pretty decent settings) except it has a better processor.