Ask HN: How can I estimate my conversion rate?

I am very near launching a rental service. I did a few $100 Google Adwords experiments, in which the ad was exactly what I was renting along with the price. I got a bunch of clicks at an average of $2-4 per click, so I know I can get clicks. The problem is, I have no idea how many of those are going to convert to sales. I'm hoping that because I'm announcing my product and price in the ad that I won't waste a lot of money on window shoppers. Anyone have any experience with this stuff? How many sales per click are you making?

Thanks for any advice, Kevin