I understand that there are a lot of varaibales between new vs established business, compitetion etc. I was not suggesting that if you increased sales you needed to increase your marketing.
However you need somewhere to start and you need to budget for customer acquisition.
I am more interested in what avenues have been successful for you and a realistic starting place.
I know this is old, sorry for my late reply, but as an AdWords professional, most of the stores I manage adwords accounts for the budget does vary depending on locality (what other people in the area are doing for adwords) and such.
But in general, I recommend a minimum of $10 a day on AdWords, and $5 a day on Facebook (FB has a $5 minimum now), and for most brick and mortar stores, I don't see the need to be spending more than $20 / day on adwords. There are tools in the adwords console to help analyze the spend and if you actually are getting the right coverage, or need to increase budget for more visibility.
Additionally, it's not a bad idea to consider ScubaBoard advertising. ScubaBoard's CPM prices are comparable to, if not better than a highly targeted display campaign on AdWords. I see that all the time.
This generalization does not apply for e-tailers. - Without getting too into it. E-Commerce strategies are different based on conversion value, and the bidding strategies can vary depending on ROI remaining positive based on tracking of conversions and dollar spends on an online store.
I see many "do it yourselfers" on Adwords making similar (to each other) choices that can be costly for their budget and effect the outcome negatively for them. The most important thing to look for in your adwords performance is to monitor your keyword performance, and keep an eye on negative keywords.