I made robot kits out of a pair of Technobots transmitter/receiver servo starter kits that cost a bit under £40 each. The resulting robots were probably a bit over 150g but it didnt matter for the application and it would have been possible to bring them in under 150 g. The only mods needed were to fix the servos for continuous rotation and to make a battery pack that takes 4 AAA to replace the one supplied that took 4 AA. The Technobot kits come with a set of crystals.