Nah, you dont need to rebuild after every race, unless you want to win a championship. I race just for fun and times. It costs me on average $50 for a day, including fuel and mainenance, and after 4-6 days, a set of tyres. They cost $230 for a set of slicks. Its good cheap fun.
Like anything though, If you want to get serious, then it costs big money. If you want to just have a bit of fun (like going to wakefield), the its cheap as chips.
Honesly, Go-Karts are sooooo cheap compaired to running at wakefield. For one trip to wakefield, you could do a kart trackday every week for a month, including a new set of tyres.
Father-in-law and I just built a trailer to haul our karts to and from the track. It cost $150 for the materials, we welded it all together and bolted it to his 6x4" box trailer. Done.