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Here's a look at the Google Traffic Estimator. It's a tool that should be used prior to making every sale, as to educate yourself on the reality of advertising costs and ad visibility.
You can find Google Keyword Planner and Traffic Estimator by hitting this link: http://adwords.google.ca/ko/KeywordPlanner/Home?__o=kta or by simply entering "Google Keyword Planner" into Google Search.
Once at the planner, you'll want to hit "Get Traffic Estimates for a List of Keywords". Here you will be able to enter your target keywords, and a geographic location to find out the search volume, competition level, and potential average cpc cost. (Remember, each search is specific to the area input, as competition is justified by the amount of people in one area advertising for one keyword)
From here, you will want to click "Get Estimates". You should go into the planner and enter in a test budget, and test bid. As a standard reference I like to use $25 as the daily budget (to simulate a $750 monthly spend) , and $5 as the bid.
From here, we will be able to see the results of our potential budget, and our potential bids. From these select keywords, it appears that with a $6 bid, and $25 daily budget, we can expect to see the above results.
Base your client's budget on these tools. If Google is telling you that your average CPC is $2.00 for example, do the math and match it up with the advertiser goals to find out what is really needed. For instance; if the client wants to bring in 600 visitors per month in this industry, a budget of $500 will not cut it; (600 x $2.00 = 1200) Therefore, a budget of $1,200 and up would be recommended.
It is extremely important to note that these are estimates and can change in the actual ad auction as campaign adjustments occur.