According to this:
http://fussball-geld.de/vereinsstruktur-bayern-muenchen-allianz-arena-muenchen-stadion-gmbh/
the Arena generated 46-54 Mio turn-over the last couple of years (variations due to CL final etc.). The costs actually were on a similar level, making the profit between -3.7Mio to 2.2Mio, depending on what year you're looking at.
Among the cost are depreciation at about ~20Mio, as are (were) the interests, which will be gone now.
So bottom line, it is mostly the money they paid for interest so far that is being freed, around 20Mio. I'm sure a huge part of it will be invested in infrastructure, there will be a new youth centre built at the old Fürst-Wrede-Kaserne.