Back of an envelope calculation, assuming a CPC of $1 and a conversion rate of 1% (say 10% download your app and 10% of those decide to purchase) it'll cost you

Back of an envelope calculation, assuming a CPC of $1 and a conversion rate of 1% (say 10% download your app and 10% of those decide to purchase) it'll cost you $100 to make a $30 sale. You can plug your actual numbers in to give you a better estimate though. For search have a look at the data, use google adwords tool to figure out how many people search for the terms you're targeting and check out how competitive those terms are. Remember even the top ranked search term only gets 20% of the clicks from that search and it drops rapidly as you go down the page.