My prices seem reasonable to me and they stand as is. I have done the work and laid it out for you, if you care to make your own then you can do so. $10 for parts and $10 to put it together for you is fair in my eyes. If you have your own car, then you already understand pricing on the cars. Add them together and total cost is about the $30 as you say, but this is just for the hardware.
As for the software, the key that makes it work, my creativity for this piece is easily worth the $20 I am asking. Or you can spend the time and effort to write your own. It's your choice!