Campaigns can be designed on a pay per click model or a pay per display mode. Google Ads are normally set up on a pay per click while Facebook ads usually follow pay per display mode.

As a rule of thumb and just for illustrating purpose: on an pay per click campaign, one GBP equals one click. This means that google will show you add every time a client searches your keywords and charge you only if the client clicks the add.

This example illustrates the cost per click model and the relation one GBP equals one click is valid an approximation that would vary based on the demand of the specific keyword. Furthermore, not every person that will search for the keyword is a client. This is why it is very important to design a targeted campaign that avoids extra-costs derived from poorly configured ads.

ROI is usually assessed with a funnel analysis. Suppose that your add is shown 1000 times, 10% of the times it was shown someone clicks on it. This means an Add Hit Ratio of 10%. If among those 100 potential clients, ten ask for an estimate, then the Visiting Hit Ratio would be 10%. Lastly if among those 10 potential clients who ask for a quotation, one buys a product, the Tendering Hit Ratio will be, again, 10%. 

Depending on your range, campaign mode and the cost you can asses estimated campaign ROI.

Let suppose a Pay Per Display Mode with an average cost of 1cents (including campaign configuration) per show. The cost of the campaign would be 10 GBP and the compounded Hit Ratio would be of 0.1%. The advertising for gaining the sale would be of 10GBP. If you add this cost to the cost of sales, you may be able to easily understand if the campaign is profitable.

Yes, all campaigns are configured with a monthly spend limit that can be customized to match any budget. Once the campaign runs out of budget, the ads will stop showing.