1. Arduino Uno - get an eBay knockoff for around $15 shipped.
2. Pololu round robot chassis kit - $24 at http://www.robotshop.com/pololu-round-robot-chassis-kit-solid-blue-2.html
3. Pololu dual 1A motor driver - $8.45 at http://www.robotshop.com/productinfo.aspx?pc=RB-Pol-110&lang=en-US
4. Protoshield - $14.95 at http://www.robotshop.com/productinfo.aspx?pc=RB-Spa-303&lang=en-US
5. HC-SR04 Ultrasonic sensor - Buy these on ebay. You can get them for around $4 apiece instead of the $30 Parallax charges for their PING sensors.
6. Hookup wire and solder.
7. Battery holder for motor batteries. I bought a 2 AA pack from Radio Shack for a couple of bucks.
8. Two AA batteries and one 9 volt battery.
9. Various nuts, bolts, zipties and pieces of wood,plastic, etc. I used parts from an old erector set. Whatever you have laying around will be fine.
10. Optional - I added a big diffused green LED and a piezo buzzer to my Clusterbot to pimp it out a little. Buy a few noisy and shiny things to make your bot more interesting.
The Pololu round robot chassis referenced above contains the motors,gearbox,axles,wheels,tires,caster and round plastic chassis. This is a pretty awesome deal for a first robot. The kit actually comes with two casters too.
The protoshield is not essential - you may substitute your own breadboard instead. I used a protoshield because I had one sitting around. It does make your design a little cleaner however.
If you already have an Arduino and either don't use or already have a protoshield, you can build Clusterbot for around $35. Buying everything listed above will come to around $70.