i just thought i'd share this with everyone else if you weren't already aware. you can VERY easily make your own top-entry litterbox for $10 or less as long as you own or can obtain a box cutter. the end result is similar to:
get a rubbermaid/sterilite container from walmart, target, etc. i use the 18L (or is it gallons?) ones. they're about $6 for a sterilite one, but you can get whatever size fits your space. get a box cutter, cut a whole (or rectangle in most cases) in one end of the lid and VOILA, a cheapo top-entry litterbox.
my 17lbs cat has no problem walking across the lid even though it's thin plastic. attach a litter collecting mat and put the end w/ the opening towards the wall, and they're forced to walk across it when they come out. the best part is that if the box gets really nasty b/c you've been lazy, just toss the whole thing out and start again. it's only $6
after seeing the one pictured above in my local petsmart, i realized how much of a ripoff it was (it was $49 in-store), but still thought it was a great idea, and made my own. i've used it for 6 years until now...i just got a cat genie last month. i still have one as a backup though.