I really wanna get into it! But when you say spend money, what do you mean?
You get the basic package as part of the subscription which allows you to install the sim and run the rookie licence races, if you want to stay here then you don't need to spend anymore money, there are championships involved with it and time trials just as with any other of the events.
Currently that includes.
Road Racing :
Cars: Mazda Mx5 (2 versions), Pontiac Solstice (2 versions), Spec Racer Ford
Tracks : Lime Rock Park, Laguna Seca Raceway, Summit Point Raceway. (+ Okayama next week)
Oval Racing
Cars : Legends 34' Coupe (2 versions), Street Stock.
Tracks: Charlotte Motor Speedway, Lanier National Speedway, Oxford Plains Speedway, South Boston Speedway, Thompson International Speedway.
Those cars you race either to compete in their respective championships, simply to earn your D racing licence or both. Some people are happy running those championships and they are fun but to move to faster cars and bigger series you must buy the content.
Example.
D Licence series ( You must run D licence events in order to move to C licence). There are 3 possible racing series you can enter in Road racing to allow you to move up, all of them require you spend money (real money) to purchase the car to run them.
iRacing Mustang Cup :
Car : Ford Mustang FR500S - $11.95
Tracks : 11 races, you need to run 8 races to count points for a full season, 3 of the road tracks you already own are on the event, so you must purchase a minimum of 5 tracks to complete a season.
Full circuits cost $14.95 each and smaller speedways cost $11.95 each.
If you're a clean and safe enough driver and you progress to the C licence with the mustang and want to progress further you then need to buy another car, and more tracks in order to compete in the C licence based events.
This is why iRacing is expensive, very cheap to start and it is a great service, the best sim around and by far the best online racing, but unless you're happy driving the MX-5 (which is great fun by the way) or other rookie races then you're going to need to spend money. The more you progress, the more money you need to spend. You may just want to race the F1 car, but you cannot get the licence to drive the F1 car without racing the lower series cars first, which means you have to buy all of those cars.
Without the right licence you can only use your higher licence cars in test sessions and (next patch) in the future practice sessions. But no time trials qualifying or races without the licence.