eBestShop4u via eBay offers the Star Master Planetarium Projector Light in Blue for $8.95 with free shipping. That's the best price we've seen for any such planetarium projector. It projects "thousands" of stars and a moon onto the walls and ceiling. The projector is powered by an A/C adapter or three AA batteries. (Neither are included.)

Update: A reader notes that 13deals offers it for $6.99 with free shipping.