There are many factors that should go into setting a B2B marketing budget. These factors include, but are not limited to, things such as the size of your business, the market you compete in, the spending levels of your competition and both your long-term and short-term strategies. If you are looking to grow your businesses size long-term it may make sense to invest more heavily in marketing at the start and then once it reaches the size you desire or that you have maximized capacity you reduce the marketing spend to a level that will only maintain sales. On the flip side if you find you are not able to fill all your orders in a timely manner you should either reduce your marketing spend or invest to increase capacity. To help set your marketing budget it is always good to look at what the industry does as a benchmark.
Here are the averages of marketing budgets as a % of gross revenue for various types of B2B companies in 2010
Small Businesses (under 100 employees) = 11%
Medium Sized Businesses (100-1,000 employees) = 7%
Large Businesses (over 1,000 employees) = 5%
By Business Type
Technology Companies = 8%
Industrial or Manufacturing = 5%
Service Companies = 11%
Source: 2011 B2B Marketing Benchmark Report - Budget Basics
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Because I believe in the long-term benefits of marketing I would suggest that ever business should invest to the market average marketing budget levels, if not more. Marketing is what delivers long-term sales increases and increased business value, therefore it should get as much spending as you can manage while keeping your profit levels satisfactory. This is assuming you have capacity for additional sales. You don't want to be increasing sales if you will not be able to fill additional orders. If you are spending money on marketing but not seeing results it just means you need to change what you are doing and test multiple things until you find something that works. Marketing investments should operate as a continual learning process where you are always looking to improve your return on investment.