I know that Xavier was rich, very rich:
- He ran his own school for years
- He built Cerebro
- He owned an exotic aircraft (possibly worth hundreds of millions of dollars)
- He rebuilt the mansion, including the contents (even if the X-Men supplied the labor)
- He (probably?) was held responsible for the damage his team did to the city
How could he afford it? Was there any other person or organization also funding his activities?