The minimum advertising budget per month is $150 (or the equivalent in your currency).
When you install Brainity your store's data is used to calculate a customised, conservative monthly budget suggestion. But this budget is only a suggestion to get you set off on the right foot if you are not sure how much you want to spend.
An additional point to consider, higher budgets often allow you to gather more data, faster. This means your campaigns will be able to optimise more quickly and begin spending your budget more effectively, faster.
(For more information about how your daily budget is calculated read here)