Simply set a monthly budget on your campaign so you don’t overspend. Yes, each keyword has a different cost per click, however overall you will only spend the budget you set. Note: When you set a budget, Google sees a month as 30.4 days, so £100p/m month budget would be set at £3.29 per day if you advertise 7 days a week.
Significant upgrade 4th Oct 2017
In the above example a £3.29per day budget is quite low and means you could miss out on high quality leads, however Google can now over spend by double the budget on a specific day if it believes the click will benefit your business. Important, you will not over spend your monthly budget as it balances out on the quieter day.
As such a £100p/m budget can now trigger keywords with a high cost per click than they previously could do which is great news for small businesses with a small budget.