From my handful of college business classes, the cost to acquire a new customer is called "Customer Acquisition Cost". Let's say you spend $1,000 advertising to get 20 customers, your CAC would be $50. So long as you get at least $50 out of each customer, you will break even on your advertising.
What you need to do is take a look at how much each new customers brings you in sales. Do a CONSERVATIVE average, NOT your best case average! Then give yourself profit of this average, and the rest can be your CAC.
So for me, an average customer will be at least 1 hour of work (minimum 1 hr charge) at $45. I give myself 50% of this as profit, so I put my CAC at $22.50.
For advertising, you have two metrics...RESPONSE RATE and CONVERSION RATE. A response is when someone emails you, visits your website, or calls you because of your ad. THEY ARE NOT A CUSTOMER YET! You only get to count them as a customer when they CONVERT and actually utilize your services.
For example, you spend $1,000 on advertising. R-Rates will vary by ad media type, but usually 3% is a good conservative number. Now let's say 10% of responses convert to a customer, again a conservative-ish number.
So your $1,000 gets you a big color ad in the local small-town newspaper. The paper is seen by 25,000 people. 750 people will respond to your ad, and 75 will end up becoming a customer. Your CAC is $1,000/75 or $13.33. You end up earning $45 per customer $3,375 in total. A great result for your campaign, which means that the rate percentages might still be too "best-case".
Then there's retention rates, but that's for another time.
The main thing here is that if you do homework before advertising, you can really gauge if you have any hope of making money off the effort. Industry-average response/conversion rates can be obtained through some internet searching, and then you run the numbers from there.
My advice is to always use numbers that are borderline "worst case". Sometimes a response rate can go as low as 0.001%, or lower, and a conversion rate can be the same. There's a lot of research into ad design to improve those rates, but again internet searching is your best friend.
Good luck!