Buying new customers

The best way to think of marketing is that of buying new customers. If you spend $1000 and get 10 new customers then it cost $100 for each new customer. Whether that's profitable or not depends on how much you make on an average sale and how many times an average customer reorders. 

If you make very little on each sale and your customers don't tend to re-order then you can't spend much of anything to get new customers, but if you make a large profit on each new sale you may not be spending nearly enough. 

If you can buy a new customer and still make a profit then you should increase your marketing budget until you have all the customers you can service.