Have you thought about building your own chassis? It may or may not be cheaper, depending on the materials you use (and where you're located in the world), but it might be more satisfying.
You can make a real nice chassis using a bit of 1/4" plywood (cut out circles using a jig saw), some thin threaded rod and nuts (maybe some washers, too); add on a couple of motors with wheels, like these:http://www.allelectronics.com/make-a-store/item/DCM-351/24-VDC-GEAR-MOTOR-W/TURNTABLE/1.html
(attach them with some right-angle brackets) - or use some plastic lawnmower wheels coupled to a pair of these:http://www.allelectronics.com/make-a-store/item/DCM-441/12/24-VDC-GEAR-MOTOR/1.html
...more fabrication needed, though - but it comes with a bracket!
Add a few cheap swivel casters...
Basically - you could end up with something more like a house robot; something that stands about 2 - 2.5 feet tall - with three or four plywood circles, the bottom one could hold your batteries and motors (and maybe motor controller); the center one could hold your processing unit (Arduino, small PC, etc), plus maybe a ring of ultrasonic sensors; the top one could hold your camera, pan/tilt unit, etc (plus other sensors). It would actually be the perfect expandable platform (leave enough room on the center or top level - or add a fourth level - and you could later add an arm, for instance).
If you shopped for the parts right, you could easily build such a robot chassis for $50.00 USD or even less (shop for some of the parts at thrift stores and such, for instance - how about this: use aluminium pizza trays for the chassis? I also am always finding 4-packs of small casters at thrift stores - people buy cheap furniture that comes with them, and don't use them).
Or - do the same thing, but go smaller. Use smaller 6-8 inch threaded rod; use hobby plywood from a hobby shop for the body (or purchase from the dollar store cheap pressboard or plastic clipboards, and cut them into circles - also, from Costco and similar membership warehouse stores that cater to businesses, they sell cardboard circles for pizza makers! Plus cheap metal pizza pans!); continuous rotation servos for the motors. For wheels, most hobby shops sell wheels that will attach to servo horns - or you can make some out of plastic screw-lids from peanut butter jars (add a rubber band for a tire). Cut a ping-pong ball in half and attach it for a cheap "caster" wheel (works on most smooth table-top surfaces).
Fabricating it yourself will open up a world of possibilities and let you make your robot how -you- need it to be. Something you need to start doing when you go out shopping or scavenging (come on - some of the best robot making junk is thrown away!) is looking at something in a way that makes you go "hmm - how can I use this on a robot?"
You'll go grocery shopping, and find items all over the place (of course, the best is at a hardware or auto-parts store) that you can use to fabricate robots with. If you have a significant other who's with you, they may think you're nuts - and maybe you are (or will be) - but that's half the joy! Also - as I noted above - be sure to check out thrift stores (my favorite is Goodwill Stores), as well as garage/yard/tag/boot sales (whatever they call 'em in your neck of the woods).
Oh - also - be sure to check out Ham Fests - sometimes you can find interesting bargains (I've bought working 24VDC matched wheelchair motor pairs with wheels/tires for $50.00 - you -will not- ever find a bargain like that most elsewhere - except at Goodwill - where I recently found - but didn't purchase - a Pride mobility chair with differential steering for $20.00 - but it needed batteries - it was too heavy for me to lift/move, too).