1) If it's to drive traffic, what is the traffic goal? Assuming one click equals one visit, you'll get the number of clicks you need. If you have history data of your ads performance on Google Adwords, you'll get the average CPC. It times the number of clicks equals budget. If you don't have history data, you may need to run a test campaign.
2) If it's generate sales, what's the average conversion rate on your website? Then, you can reverse calculate to number of clicks.
3) ....
Besides, you may need to consider other factors, e.g. your competitors' search engine campaign,etc.
