Problems: The only problem I had was the photocells because when they received light reflected off the floor they just kept running even when they got on the black line but I fixed this by adding the interval timer.
Parts: 1 Twin Motor Gearbox Kit $19.95 from www.robotstore.com
1 Narrow Tire Set (pair) $9.95 from www.robotstore.com
1 Interval Timer Kit $9.95 from www.allelectronics.com
16 AAs $4.00 From The Dollar Tree
1 Rubber Band $Free
22 Guage Wire $Free
1 PC Mount DPDT Switch $0.25 from www.allelectronics.com
1 Panel Mount SPST Switch $Free
1 Grif-Style PC Board-356 Holes $1.79 from Radio Shack
1 Grid-Style PC Board-750 Holes $3.29 from Radio Shack
2 Metal Standoffs With Screws $1.59 from Radio Shack
3 4 AA Battery Holders 3 Times $1.69 from Radio Shack
2 2 AA Battery Holders 2 Times $1.49 from Radio Shack
2 5 ohm light, 20 ohm dark photocells $Free
1 Krypton Bulb $2.59 from Radio Shack
2 5vdc DPDT DIP Relay $1.50ea. from www.allelectronics.com
1 6x3x3" Project Enclosure $3.79 from Radio Shack