Re: What happens to surplus budget?
I'm going to assume your account is prepay, and you are paying $100 upfront in your example. If you only spend $80, you'll still have $20 left to spend next month or 3 months later. If you decide you don't want to advertise any longer, you can request a refund for the unspent $20. More details can be found on that here: https://support.google.com/adwords/answer/1703646?hl=en