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The answer to your question is to determine what's called the average lifetime value of your customers, which it looks like you have already started doing. There are a variety of methods to calculate the value, but the easiest is to divide the total value ($700) by the number of uniqe customers (40) which give you a lifetime value of $17.50 per customer you convert through PPC. You can calculate ROI using this value. BUT do not use this value for your book keeping! Lifetime value can advise you how much your customers are worth and how important it is for you to keep them, but you still have to retain them through service, quality products, communication &c.