For top players, it's the organizer who foots the bill just to get the name value. For some countries, they'll pay for their players to travel to a well-known tournament to show the "greatness" of their country. The less famous tournaments couldn't afford a top 20 GM, but have been known to "bribe" lower GMs, hoping other GMs ...
For invitationals generally the organizers will pay (or the invited players won't accept the invitation). For open tournaments it's a different matter, and the players themselves pay.
An event like "Serbia open masters" might be funded by Serbia's chess organization, though (I have not checked).