Radio Button to fill in Text Field

I apologize for probably asking the same question that someone else has probably asked. I have searched through the forum and tried many different scripts but I have not been able to get this to work for me.

I have created a registration form. I have created 3 radio buttons.

1 Radio Button - #E_Reg

2 Radio Button - #L_Reg

3 Radio Button - #C_Reg

They are all in a group named Reg.

If someone selects the button 1, I need a field named Total_Reg to populate $25

If someone selects the button 2, I need a field named Total_Reg to populate $35

If someone selects the button 3, I need a field named Total_Reg to populate $45

Basically, it is 3 different registration prices. If they are early, they get $25, if they are late it will be $35, if they wait until the convention then it will be $45. Only 1 can be selected, so that they are charged appropriately.

Can someone please help me?

Jason