A cheaper alternative is to see if they have open practice days, instead of shelling out thousands for a private event.
The kart tracks here in California charge around 30-50 bucks a day, per person. You might have to split up your time with karts and super motos, but call ahead and ask what the off days are.
I've been to practice days where there's only a few other karts and bikes, and spent plenty of time on the track.