material
one of the 2 probably,,, polyester or polycarbinate
But to keep them cheap id hazard a guess @ polyester ,,
i work with the injection moulding industry and this would be one of the materials id probs go for...
Have now looked into this further and yes ABS OR POLYPROP,
ABS Is an impact modified material making it strong but prone to cracking etc , polyprop is a very durrable material and will give more than ABS .