It is very expensive for their pride.
Those billionaires don't actually have the money ready in cash, just a tiny part of that is liquid, everything is in stocks. They are getting paid in stocks from their company. Why? Because until you're selling the stocks, for the government you don't have a revenue to tax. Suppose this year you get a salary of $100 billion in stocks. If you convert in cash only $100k, then the tax owed that year is minuscule. Those assholes have teams dedicated to minimize the amount of taxes owed. It is why he didn't use his money to buy xitter but used his stock and "genius CEO" fame as a collateral to persuade banks and other arab investors to lend the money for him (and hold the bag if it sank)
So in order to donate that quarter of a billion he had to liquidate stocks and pay taxes on that. Ouch, so so much painful for a billionaire. For them paying taxes is like being robbed. So, it's more like paying $20k for you