I picked one up off FBM for $50 - hardly used, looks like new. My Grandson just turned 1, so I have a little time to upgrade it before he's ready to drive it. Cut the sides of the tailgate so it will open like a tailgate should, got the hood to open and will use that area as a small storage compartment, added an additional motor and gearbox to the other rear wheel so now it will be at least 2wd, upgrading to 12v for now, maybe 18v in the future.
For you guys wanting to experiment with seriously upgrading the drivetrain (like 24v, 36v, 48v, etc) check CL and FBM for "Razor" bikes and scooters - you can pick them up for $50 all day long. Hefty motors, freewheeling drive hubs, disk brakes, coilover shocks, controllers, chargers - lots of useable parts on those things. The batteries are usually dead, heavy lead-acid, but new batteries (even LiPos) are not too expensive. Sure, some fabrication will be required, but that's half the fun.
Back when my son was small (like nearly 30 years ago) I picked up a "Bigfoot" Power Wheel for him, and went to work modifying it. Fabbed a trailing arm rear suspension with old liftgate gas shocks and hardware store springs for coilovers, used more springs around the front spindles for some compression damping. Converted it to 12v with a trolling motor battery mounted under the seat, it would run for hours. He quickly discovered if he was going in reverse, then flipped the switch to foward while keeping the throttle pedal down, the sudden weight transfer would produce wheelies. It was so cool to watch him get in it - he'd step on the rear wheel, then onto the seat, the suspension would compress a little, he'd plop down in the seat, stomp on the pedal, and spin both tires taking off.