I just got one from ebay UK (you don't say where you are) - £10.49 inc postage - Standard Black Sony used but you can't tell. This is the one I am using all the time.
I'd had already got a Sony lighted cover again from ebay (Argos Clearance for £22 inc postage), but wanted something smaller as the light can be a nuisance when you are not using it. You can remove the light section but are then left with a bigger case than required. I still find it preferable to use a 'normal' light where possible.
I since got 10 Sony Silicone cases (£11 for them all including postage) again on ebay! I actually prefer holding the PRS-505 in this to the others, but it doesn't give the protection to the screen when you aren't using it. I cut the corners out of one of these to fit it as well as the leather case, but it distorts the hinge mounts in the case as there isn't enough clearance for the thickness of the silicone.
I didn't actually get the tan case with my PRS-505 as it is made up of two PRS-505's from guess where
and a new screen from China!