It depends on what classes run where you want to race. LO206 is a very competitive class with lots of entries at my local track. It has a very stable rule set and emphasizes the driving and setup aspects of racing. I personally run World Formula as I am a bigger guy and the extra HP helps, once again this is a reliable and relativity cheap class to run. By watching what I spend and shopping around I have less than 3K in my total program. That includes Kart, Engine, tires, tools and trailer. Ongoing costs after getting started are track membership, entry fees, travel, and maintenance of equipment. It all depends on how serious you want to get, me I just do it for fun and the great time I have with the people at the track.
