Changing UI.TextField value one character at a time

I’m working on a price calculator application and I'm trying to implement a feature where by default you see $0.00 as current value for the price UI.TextField, but when onChange event triggers. I want to replace current value to user generated value. So, let’s say the first value user typed is = 9. Then you would see $0.09 and if the next is = 2 then you see $0.29 etc. It would basically replace 0 with new value One character at a time and skip “.”. Any suggestions will be greatly appreciated.