Depends on how many prebuild (proprietary and expensive) components you want. I think the absolute best platform for robotics is Arduino. Pretty much anything

Depends on how many prebuild (proprietary and expensive) components you want. I think the absolute best platform for robotics is Arduino. Pretty much anything you can think of has been done, and there's likely a step by step tutorial out there somewhere with component lists. Arduino UNO clones can be had for as little as $3-$4. And the components like LEDs, motors, servos, etc can be had for a tiny fraction of what a lot of the proprietary systems cost. The downside comes when you want to do things like attach a motor to a lead screw, or attach something to a servo. You'll probably end up needing a drill press or an improvised lathe. But, I think compared to the cost of the proprietary systems, you can still come out ahead. And you don't have to worry about breaking something or dedicating a motor or controller to a project because they can be replaced cheaply.