Your First Robot Kit: CamJam EduKit #3
If you want a cheap robot kit, it's hard to do better than the CamJam EduKit #3. This is a small kit which you can assemble into a two-wheeled robot with a caster, as long as you buy a few extras. It comes with a line sensor and ultrasound distance sensor, plus a series of exercise sheets to work through in order to build the robot and get it running. You can optionally get your own chassis, or you can use the box the kit comes in.
I have recorded videos that cover all of the exercises given by the repository and uploaded them to YouTube. This post goes through each video and briefly explains what it shows, so you can follow along if you choose to buy the kit. I also use the box as the chassis!