As PPC consultants, we are often asked, "What is a good starting budget?" It's important to have enough budget to drive enough paid traffic to have good data. The data flow is essential to optimizing any campaign. You have to have enough traffic to determine which keywords are working well, and which ones to pause. Which ones need to be bid up, which ones need to fall away. All of these decisions are made by the data.
Another important issue is comfort level. You MUST commit to the pay per click budget for at least 30 days. We can not play the paid search budget yo-yo game. Leave the budget level so we can drive as much PPC traffic as possible and get as much data as possible.
So, where is this magical number? My answer is about 20 clicks/day for each campaign. Take a look at the keyword tool to see what the predicted CPC may be (this is never perfectly accurate). Take the high average and multiply by 20. This should give a conservative estimate of what your starting budget should be. So, if the average CPC is estimated to be $1.50, your starting budget should be $30/day.
In most cases, your starting budget should be $20-50/day!