Is $600 to repair a wall damaged by water a fair price?

I recently noticed the corner of this wall next to my shower looks weird. I'm guessing some water has been falling on it while I shower and damaged the paint. I'm not sure what to do. I contacted a guy but he is asking for $600 to repair the wall. I thought about doing it myself, reading some websites I understand that I should scratch it off first, put some kind of compact, and paint it. I'll appreciate any help.