The guy is the third richest man in America. He could donate $1 Billion no problem. Not that any amount of money would buy dedicated supporters, but it can be used to sway the masses who don't follow politics very closely.
Another element of these donations is that he is making a statement with them. These donations could be secret, but he is making them public. He wants this to be reported, so that people, especially insiders, know that Newt has endlessly deep pockets behind him.