Let's say a client asks for a new development/feature, how do you calculate or estimate how much to charge them? Do you do it in advance (making a budget/estimate) or do you charge for hours worked?

Most times we need to make budgets for clients, how do you do that? How do you make sure you don't charge less or more than you should?

We've been kinda struggling with this for a while and decided to check if any of you has any good tips!!