It's really not too bad. To buy a 4WD all-terrain kit from somewhere like SuperDroid Robots costs around $1500-2000 without any kind of automated control system

It's really not too bad. To buy a 4WD all-terrain kit from somewhere like SuperDroid Robots costs around $1500-2000 without any kind of automated control system. That will get you four motors with encoders, batteries chassis, wheels, a good (Roboteq) motor controller and an RC kit. The one we had at work could easily drive with a large adult (i.e. 80+ kg) sitting on it. I mention SuperDroid because their chassis are high quality welded aluminium, chain driven wheels etc. They sell to hobbyists (robot wars) and industrial markets including law enforcement. Most of the markup is on the frame. If you have acess to a welder and something to cut aluminium you could do it a lot more cheaply. And yes, high quality motors with encoders cost $150 each. As always, you can go cheap as a hobbyist and pay with your time. If you want something semi autonomous you can buy a Clearpath Husky for about $20k.