SIM racing offers an affordable avenue of racing with cost ranging anywhere from a budget setup of $500 to full professional $100,000 setup. Because it is one of the least expensive forms of racing the competition (top 10,000 competitors) is literally separated by less then a second at some tracks. You'll need at minimum a tv/monitor, steering wheel, pedals and gaming console or pc. A gaming PC will give you many more options.
A great way to get started is with your racing video (sim) games then add a steering wheel and pedals. Generally speaking the higher quality you go with your pedals and steering wheel the higher level of similarity between off track sim racing and on track racing.
Karting can range from indoor karts with speeds around 25-50mph to Super Kart shifters with sub three seconds 0-60 times and topping out near 150mph. Indoor karting cost around $20-$60 a day roughly where shifters can cost about $15K-$25K for a competitive kart, plus your hauler, safety gear, tools, spares, tires, and travel fees.
One the most demanding sports on the planet. Normally if you race motocross there will be injuries however it is a very amazing experience to float through the air like an eagle on top of a 200+ lbs fire breathing piece of machinery. There are generally many more tracks then karting and more used bikes to soften the entry into the sport.
Auto racing is going to be the most costly form of racing ranging from simple front wheel drive stock car racing as probably the least expensive to open wheel racing in Formula One with teams spending almost a half a billion a year to run near or at the front.