Aggressive advertising from competing businesses especially large retail chains make it necessary to include an advertising budget in your yearly business plan.
A general rule of thumb among experts is that at least 1% of your gross revenue should be allocated each year to the promotion of your business. However in many industries the figure can be as high as 20%.
If you are just starting up or have a plan for growth for your revenue then you need to calculate 1% (or whatever % you decide) of the revenue level you want to grow to.
Advertising and promotion is an investment in the success of your business. It should not been seen as a cost without any accompanying benefits. An effective advertising program should in the long run eventually pay for itself with increased sales.