The trick is to choose very targeted keywords that will trigger your ad
(yes, you can do that). Which means that only very targeted buyers will ever
see your ad and your conversion ration will be incredibly high. You can set
how much you wish to spend. Google takes the money from your credit card
after you owe $50, by which time your ad will have been displayed roughly
5,000 times. If your keywords are highly targeted, many of the people who
see your ad will become buyers and you will get your return before you even
pay Google!