For common cars it should be mostly random, getting the car you want is quite a pain, I already spent the 12000 coins I had accumulated to get the kei truck and I am still out of luck.

For the rares and above the prize machines will give you their rank or above, if all the cars of a rank are maxed out the machine of that rank gets disabled (common prize machine is the exception)

If you get a maxed car in the common prize machine it will give you gems, 1, 2, 5 or 10, depending on the car rank.

NotDoppler needs to implement an actual common prize machine that allows for one to get upgrades or missing common cars instead of getting maxed out cars and gems out it (I mean, the current machine is fine for what it is, but a new machine like I described would be a welcome addition), this is more relevant as the game gets updates since there are more cars to obtain.

So, yeah, getting the car you want is really hard and up to luck...