While I agree that harder difficulty levels should be rewarded more, I think that the race length is the thing that should net you the greater rewards increase. If the race rewards were only based on difficulty and length however, you could make your first races long and difficult and if you beat those you could then get a supercar straight away, which would destroy progression. You could limit the selection of available cars and parts by the player level too to remedy this, but I think money is a better guide. I think that a racing game should start from the grass roots with a low budget, like in the old GT titles. They give you 10-20k and you start with whatever car and upgrades you can buy with that sum. At the same time they are given a look at the car selection in the game for the first time.
Instead of just difficulty and length scaling, I would like to see the main chunk of the price money to come from the player level or tier or whatever it will be called in the end. This way the player would be forced to start with a selection of cheaper cars and decide if they want to spend money upgrading them or just buying new ones, or using the inevitable reward cars instead. At least for the beginning, until they rank up enough to start getting high enough price money that easily gets them super or race cars. This scaling would globally rise the price money for every race, making the skipped events worthwhile even later on.
I’m not saying that the players should be forced to stick to boring slow cars for hours on end, but I’m saying that there should be a visible progression in the car lap times for the early game. Start with a cheap car, it gets upgraded into a faster road car, it then upgrades into a supercar and finally to a real race car. Once the player has unlocked high enough level of speed, the “training wheels” could be removed and the price money and car rewards be increased so that the players can start doing whatever they wanted.
I dislike games that drown me in free cars in the beginning. I would like to buy some of them myself. I think a better way to encourage players into progression is to not give them an SUV for that upcoming SUV race, instead give them enough money to select the one they like from the selection they can afford, or use the money to participate in some other event. It sucks to be in a position where you don’t get rewarded enough to buy a car you like for the next event, and instead you are encouraged to use the car the game gave you at some earlier point. Sure the player could farm money, but it has to be within reason. Looking at you, GT7.
Player should be rewarded with a car for a completed event, but I think that car should be something fitting to that, instead of being a gateway directly to the next event, (looking at you again, GT7). GT3 did this a lot, for example giving a Zonda race car from the Zonda only event, and so forth. FM might not have one make races like my example, but I think the events still should reward the player with something fitting to the it.
As a last mention, GT3 had this, I think it would be fun to have some popular events have a car price pool of more than one car, requiring multiple completions of the championship to earn them all.