Problem is, it's simply the wrong question to ask.
Your advertising expenditure should determine your sales volume. So, if you decide you want to double sales, you simply double your promotional spend. If you can't measure the relationship between advertising expenditure and sales, it's either because you don't have a system in place to make such measurement possible, or because your advertising simply doesn't work. Either way, building a system to measure the effectiveness of your advertising should be your number one priority. To do this you need to record a 'source' for every sales inquiry your business receives.