It can be a little overwhelming, but running ads on Google is a critical part of most small business strategies. It's hard to come up with an average budget because the cost of running ads depends on a lot of factors, but small businesses can spend from $1,000 up each month. Businesses that have customers with high lifetime values can spend up to $30,000 per month on their adwords budget. And many businesses hire someone to manage that budget, usually for a percentage fee of their monthly spend.
This article goes into more detail.