Turtlebots are nice, and the newer ones are not too expensive. The Turtlebot hardware is very capable and fairly reliable, so it's a good platform for software experiments and testing new sensors.

If you want something a little cheaper, some of the older Neato vacuums make a nice base, and you can attach a Raspberry Pi to run ROS for SLAM. They're a bit more limited because you have to add your own battery for the Paspberry Pi and you're limited to the lidar that's already part of the Neato.