In the US, citizens are only allowed to donate $2500 per election to a single candidate. You can of course donate as much as you want to a Super PAC, but this money would not be directly available to the candidate. However billionaires like Donald Trump, Michael Bloomberg and Tom Steyer have spent tens of millions of dollars to jumpstart their own campaigns, far beyond the ordinary $2700 limit. This is currently legal in the US.
How common is the lack of restrictions on self financing in other democracies?