I just got addmiision from CWRU and a professor said he is matching me with offer. I asked, how much I can get per month if I was a RA
I don’t understand what you are asking. You cannot be an RA first year and you have to go through a lengthy, often unfruitful application process. I don’t know what “professor said he is matching me with offer” means either.
Do you mean a Research Assistant? Or a Residential Advisor?
Are you a grad student?
You should email the professor thanking him or her for the offer and asking for more details about the position. Ask if there is a formal application process you need to go through and ask about the expected hours and compensation. Don’t just ask about the money, ask about all the details including a formal job description.