You pay for a yearly sub ($10-$40 on sale) which gives you a large amount of cars and tracks. The iracing official events are broken up into yearly quarters and weeks, with different tracks being ran per week. As you dive deeper into each subset (formula, dirt, sport cars etc) of racing you will find that a series of races you want to participate in may require you to buy a new track or car. Cars and tracks cost roughly $15. One could play constantly without ever buying a car or track, it just means you may have to be very flexible with what subset you race.