San Francisco is covered with billboards for Brex, a startup that provides corporate credit cards to startups. Plenty of startups find opportunities to brag about spending so little money on marketing, but Brex cofounder and CEO Henrique Dubugras says the ads were worth every penny. Brex, which has raised a total of $57 million in funding from Peter Thiel and Y Combinator, spent only $300,000 on the ads plastered on bus stops, buildings, and billboards. In San Francisco, there are few places to escape them.