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First of all, you don't need MacKeeper. It is a SCAM and also kind of malware. So if they call again, tell them you are not interested. If you have MacKeeper app, delete it.
You can find the price of latest Macs from here --> http://www.apple.com/mac/
(Choose the Mac model at top in the above site and click Buy to see the prices)
Re cleaning, you can erase the Mac's hard drive and install a fresh OS X on it. To give you instructions, I need to know the version number. What version of Mac O S X are you using ? You can find it from Apple > About This Mac