How it works
It is charged as it happens: per click in shopping and search campaigns, per impression in awareness ones. The daily budget caps the spend but does not guarantee it: with no demand the money simply is not spent.
Example
The same shop: 8,000 clicks at an average price of 5.
Spend = 8,000 × 5 = 40,000. The daily budget was set at 1,500, so the ceiling was 45,000 — less was spent because that was all the demand there was.