The EB website is currently running a clearance sale on Prophecy Advance. The excellent Gameboy port of Wing Commander Prophecy is availably for just $4.99. GameSpot Complete members get it for $4.49 and shipping starts at $1.99. You can see if copies are still in stock here.
The sale has been mentioned at CAG, so quantities may be extremely limited. If you visit and receive an "Out of Stock" error message, just return later and try again. New copies apparently reappear periodically. If you can't get the EB sale to work and still want a cheap copy, Half.com has new copies starting under $9.