small vibrating motors: ~84c each (ebay - $8.36/10)
LDRs: ~5c each (ebay $0.99/20)
Simple SPDT switches, ~10c each. (ebay $1.86/20)
Toothbrushes : ~17c each (dollar store $1.00/6)
2xButton cells: 25c each (dollar store $1.00/4)
And creepy crawly to go on top......

Originally I didn't know exactly what I would use, I was at the dollar store for the batteries and toothbrushes and initially picked up a bag of "creepy crawlies" that had roaches, millipedes, spiders etc in various colours. The problem was that they were very cheap (a pack of 32 for a dollar) but looked cheap and nasty. They would be big enough from above that they would cover the bristlebot, but they were flat and would look like they were an inch off the floor, and then when you looked even slightly from the side you would see the bristles and everything else.

Then I found these rubber rats and they were perfect, they are hollow inside and I figured if I used 2 button cells instead of a coin cell in each one, I could hide everything in the hollow and they would be even creepier.

Rats: 13c each (dollar store $1.00/8)

A total of about $1.80 per rat (or it would be if any of these could be bought individually or in similar size packs)