Each auto-boosting rule creates a new campaign in your Facebook ad account and stores all posts that meet rule's boosting conditions in new ad sets with one ad inside. When setting up Auto post boosting rule you run into budget settings. There are 2 types of budgets: Daily and Lifetime and 2 levels for setting the budget: campaigns and ad sets.

If you set the budget at the campaign level - it means you pay $10 daily for all posts going to get boosted simultaneously. The number of promoted posts is defined by setting Number of posts promoted at once. 

Note, Lifetime budgets are not available at the campaign level. 

2. Budget is defined at the Ad set level.

This type of the budget is defined on Ad set tab and allows setting daily or lifetime budgets.

Let's say you want to to promote a certain number of posts at once, let them run for 2 days and pay $10 for each post. 

This way the system will do its best to spend $10 every day on each post going into promotion. 

Another option here is applying lifetime budget. Having settings as on the screenshot below you will pay $10 per post for the whole period (2 days). 

For more details on post boosting features, please visit out Help center or send a message in our app-chat if you need any help while setting up post boosting rules!