This is THE question everybody would like to ask. They actually do and I believe everybody has the right to know how much advertisement will cost them. If you started an advertising effort tomorrow, you would ask the same exact question.
The answer is not that straight forward as the amount of budget needed varies depending on some factors:
1. The objective of the advertisement (Selling 10 products per day would need a lower budget than selling 100 products per day)
2. The industry of a firm. Advertising in some markets costs more than advertising in others and therefore you should have a budget that is aligned with your market needs.
3. The country and language you would like to advertise in. There will be locations and languages where you will find less competitors and therefore get more from a small investment
4. Your personal experience. Sometimes being an expert in a given market segment will give you the advantage of knowing how to offer a service with results above the average. This will definitively have an impact on the final budget level that will be necessary to get things running
Generally, it would be smart to not create any budget expectation in clients until you have tested both the market and the campaign you created.
Search Engine Marketing involves so many factors that you would like to run a campaign for some days before giving an exact estimate of the cost.
One last tip: be always transparent and do not oversell yourself!