I have three fields in my form:

Bid Price

Final Price

Result

I need the form to automatically update the Final price with the Bid Price when the Result = "Successful".  Anyone have a script or can point me in the right direction? 

I'm confused as to whether I need to attach the script to the Result field to update the Final Price or if the script needs to be attached to one of the other fields and what the script would look like.

Please help.