Hmm so no weighting on the wheel spots? That's good news.The chances for all prizes should be equal - 1 in 9 when all fields are still available or higher when some fields are already empty.
My assumption has always been that when they create a new wheel they dump all the prizes into an array or other iterable object, and then just iterate through it every time there's a spin. Rather than generating a random number on every spin and keeping track if a slot has been won or is still available. I thought this had been discussed in a past year, but I can't find it so I guess I'm imagining things.