Using Radio Buttons to auto fill totals

Hi,

this is all very confusing to me but I think what I am after will be very easy for some of you.

I am creating an order form with x3 services options

I have x3 radio buttons, one for each service I am offering and would like a sum amount attached to each option

I would like the signer to select one option and have it auto fill the fee total field and then fill in the 50% retainer fee field automatically

I have named my Radio Buttons:

Deluxe, Cinematic, Highlight

My fee fields are named:

fee,retainer

help would be greatly appreciated, I have really enjoyed making a form for the first time but this aspect has me confused, would be great if you could click on fields and have them enter in a formula like in excel, anyway TIA