Excerpt from: Don't Waste Money on Google Adwords
You should spend as much as you need for each Google AdWords campaign to achieve your business objectives and stay within your budget. Google allows you to set a budget for each campaign so that you never exceed your budget limitations for any campaign.
To use Google AdWords effectively, you need to separate your product lines and brands into distinct campaigns. All you have to do is click to add a new campaign for each product line and brand. This will allow you to customize ads and keywords for each product line and brand.
Next, set your budget for each Google AdWords campaign. For you, it may be as simple as dividing your total Google AdWords budget by the number of campaigns you’ll manage. Or perhaps some campaigns are more revenue-generating than others, and you’ll want to budget each campaign differently.
When deciding how much to spend for all your Google AdWords campaigns combined, you’ll need to examine your own resources and what your business objectives are. The good thing is, Google allows you to set your own budget for each campaign, so you can start with a campaign budget of just a few dollars a day until you identify which ads and keywords are working and which are not.
To learn more about proper budgeting for Google AdWords in order to meet your business objectives, read Don't Waste Money on Google AdWords.