AC Compressor & Muffler Cost

[afasa09]

First time posting to a forum so if I am posting in wrong area or not using proper etiquette let me apologize ahead. My new wife's car (2009 Sentra) was sounding like it had an outboard engine running. I was worried that it was the catalytic converter because when I went to take it into a local mechanic the check engine light was on(wife happen to leave that part out), also when going down hills it felt like it was struggling to shift gears. Thankfully the mechanic said it was not the converter but still had to replace everything behind it. Price $450 for parts and labor, I thought it was a fair cost but I also have never had to replace a muffler on any car I have ever had.
My other question has to do with what is the average cost of having a new ac compressor put in. I live in MD near Baltimore. I was quoted at $900. Thought about trying to do it myself but I just don't know if I have the time. Thanks in advance for any help and also any suggestion for a first time user to writing on a forum.

[RED_DET]

Replacing the A/C will require proper tools for installation to include the extraction of the R134.