Defining a single monthly budget is not possible. Every business has different advertising goals and is competing in different markets.
What we can advice from the experience of our hundreds of clients, is to start with a minimum of $1,000/month. Lower budgets won't allow any optimisation, nor leave space & time for notifiable results.
Keep in mind that advertising budgets can be adjusted at any time. Many customers are starting small and increase their ad budget over time when they start seeing results.
Online success depends on many different factors such as your product or service, market, pricing, website responsiveness, etc. Providing a sales guarantee is therefore impossible. What we can guarantee is best-in-class marketing offering high-quality traffic, which will lead to sales if other factors standards are met.
Management fee is refundable on a case by case basis. You cannot ask for a refund of used ad credits, as they were already consumed to show your ads to potential customers.