You'll have a hard time finding a somewhat decent skimmer for under $40 new or used. Though, somebody on the boards here bought an off-brand skimmer off of ---- for something like $25 or $30. Jebo sticks in my mind as the manufacturer of the skimmer, but don't hold me to that.
JEBO makes some pretty cheap stuff so that was probably it.
You get what you pay for though, their lighting systems are cheap as anything but I'm afraid they would burn my house down