Hate to say it but I'm pretty sure if you have rust on the top of that rocker, there is rust on it where you can't see it....
If you really want to do it right, and only once, change out both. There are threads in the archives about this subject, and it even explains how to, and how to protect the back side, where you can't get to once everythings back together and painted.
As far as cost to have this done... sorry, I can't answer that from experience. I myself, will be doing this same work soon. I already have the new rockers, and all the tools necessary to complete the job, its just getting myself geared up to dig into it.....
Not looking forward to it, thats for sure.