Originally Posted by
Hornypony
You can very roughly count the position of the skill to its level to comprare skills. The actual upgrade time does seem to be rather arbitrary. It grows. But the only things I can tell rather safely: they do grow slower then the money demands and they do less than double each lvl. And since they grow slower than money requirement, I felt no urge to investigate further.
Should you plan on collecting data, do not forget to write down your game bonus. It tells you the decimals in the shop where you can press reset.