I think raptornx01 and huge_vermin have it right - The University has been there so long that it owns large swathes of property all over the city. In the modern city, quite a lot of that is rented out (Unreal Estate included), and that is probably where a lot of the University's money comes from. Plus, considering that up until recently they didn't pay taxes[1] meant they have several hundred if not thousands of years worth of accumulated wealth just sitting there. After all their biggest expense would be food and drink, and possibly building repair. I'm not certain any of the Wizards actually get more than the merest stipend despite their tenure, because all meals, accommodation, research materials and coal[2] is provided to them as part of that tenure. If an individual wizard has their own savings, what would they spend it on? Most things they need they get from the University within the University walls, and it's only stuff out in the city or the wider world that would cost things, and I'm sure any reasonable expense request would be approved by the Bursar (assuming he is bribed with enough Dried Frog Pills)
[1] Okay, technically the University still does not pay taxes, but it does make an entirely voluntary donation to the city amounting to $200 per capita per annum, under the kind agreement of Lord Vetinari. (I can't remember the exact book this is referenced in, but you'll know it if you find it)
[2] seven buckets in the case of Rincewind (see The Science of Discworld 2, I think)