I didn't see this posted anywhere else, so... Amazon has Metal Slug Anthology (PS2) for $17.70 at the moment:

http://www.amazon.com/Metal-Slug-Ant.../dp/B000HTWWUU

The PS2 version is apparently the best one. The price has been steadily dropping for the past few weeks, and it's now below my purchase threshold. It may drop some more, but I didn't want to wait.

EDIT: Now $15.93!