https://litecoin.info/Mining_hardware_comparison
You've probably seen this but this has all of the individual GPU model performances.  With Scrypt you'll typically see that the more GPU memory the more possible hashes.  To find the best possible dollar/hash amount you'd have to rely on that list for the k/hash and divide that number by the lowest price you can find the GPU at.  Then compile a list and look at the secondary variables: heating issues, power issues, manufacturer issues, etc. to determine the best buy.