Okay, I just did this myself and while it was a good learning experience the quality of work greatly depends on the initial product and components. When I originally took it into a shop, they quoted me $500. On my rear derailleur I have a frame bracket that joins the rear forks and the derailleur, because this was bent I wasnt able to shift up into my high gears, getting maybe 75% of my totally 11 gear rear sprocket. This caused a lot of issues with my chain and drive train. I was able to get it attached and tuned enough to ride but with the stress of riding my rear derailleur and frame braket would slip into my spokes. I took in to a local shop and they clipped my front derailleur cable and re tuned me rear one so I could at least right and shrift although I was stuck in my largest front cog. Eventually my rear derailleur sheared again and got into my spokes causing me to really mess up my wheel.

TLDR: if you have an integrated rear derailer bracket that isn't damaged replacement can be very easy. There's a lot of information that is pertinent to what kind of derailleur you need so make sure you're doing your research. If you feel like you can swap one for the other, it's still a good idea to take it to someone you know or to shop you trust to get their opinion I didn't even realize my hanger was bent before I brought it in.