You will find cheaper and good hotels outside city center. And Munich has a fantastic public transportation system and Marienplatz is easily reached from almost any line (U-Bahn, S-Bahn, Tram). I would try to book a hotel close to a station of the public transport.
Pls. keep in mind that there are further construction works planned for the S-Bahn during next summer which will cause S-Bahn lines to be closed down during certain days. In case you talk German, you might want to have a look into the
article from Süddeutsche Zeitung.