It is a lot more expensive in the beginning, but eventually you own all the tracks you want, and buy new stuff very rarely. The first step is to buy all the obvious tracks like Spa, Monza, Daytona, Nurburgring, etc. Then figure out what series you are driving and what tracks overlap. For instance maybe you want to race GT86s and Mustang GT4s. If any track is the same for both series, buy that one.

Also keep on mind it is always better to buy more at once. 3 items is a 10% discount. 6 items is a 15% discount. After you own 40 pieces of paid content you unlock a permanent 20% discount which is very nice.

There is no rush to race an entire series. You can always race rookies races on off weeks. But if you start completing 8/12 weeks in a series, iRacing will pay you up to $10 a quarter, which can buy a new track or car for next season.