From what I have read on the developer page, Android In-app Billing doesn't support refund procedure. You have to use the Google Checkout account in order to issue a refund.
I follow these steps:
My question is: Is there any involved cost in doing this? Or, in the end, my developer account and my testing account will both have the initial amount of money?
This is well managed by google, your client account and your developper account will both have the initial amount of money yes.
see official doc Here.
it says it at the end in the "Things to remember" note ;)