The way Google’s Adwords system works is that they post-bill you when you hit a certain dollar amount. The dollar amount increases as you build credit with them. I think they billed me when my balance hit $25, then again when it hit $50, then again when it hit $100, until finally they were billing me for every $500. So even if you wanted to risk karmic retribution by defrauding Google’s ad program, you probably wouldn’t get away with very much advertising.