We have a Diggler Scooter we bought for somewhere around $350 six years ago. I think it was their top of the line at the time but I'm sure they've made many improvements (and price raises) since then. Seemed like a hefty chunk of change even at the time. Nice product though - it's held up great. And Grace was driving us INSANE, so we really had no choice. There just weren't enough hours in the day to exercise her as much as she wanted and we lived in DC. However, with the scooter and the many greenways, we were able to coexist happily until she chilled out some. Now she's 8 and we just do it for fun. I also use it for ADHD fosters.
The dogs pull it using x back mushing harnesses and they pull from up in front (I didn't like the side mount harness things for the same reason as the above poster).
For training them, we had my husband ride his bike in front of us. GSDs are such velcro dogs they went wherever he went and ignored everything else.
I would rather skijour too but I've never lived anywhere with enough snow.