I believe this is directly affected by the price, profit margin and market of your product. I like to always keep around the 10% mark for our companies for CPA or cost per acquisition. Also, is your product a subscription or retainer based service. If so, what is the lifetime value of the client and what sort of referrals will they bring to the business?
For example, let's say the average sale per customer was $1000 and the retention was three years. That's $3000 lifetime value. What would you be prepared to pay for that customer? $100 - $300? I would be looking around the $200 mark. Then you have to understand conversion rates... Let's say you have a PPC (pay-per-click) campaign that cost $1,000 that reaches 1000 qualified people and you get a 20% click through rate with a 3% conversion rate into customers. You would get 6 new customers.
When you divide 6 by the $1000 spent you get a $166 Cost Per Acquisition.
So when you are talking about understanding the returns, you could then argue that when you spend $1000 on marketing, you will get $6000 in the first year which is a 600% ROI and then further on you can argue the lifetime ROI is $18,000 from a $1000 spend.
All in all, the best way is to test marketing mediums, headlines, video VS text... then measure what the results are.... In other words, test and measure....
I hope that makes sense.