It’s not difficult if we have all the requirements, and its’ fool hardy to give estimates without detailed requirements. Think of it in these terms. Would you ask a contractor for an estimate on building your dream home without giving him any architectural plans? What would be your confidence level that he would understand what the house is suppose to look like and that it would include all the features and aesthetics you wanted? What would be your confidence level that the estimate he gave you would be remotely close to the actual cost?
Without doing the work of gathering requirements, you only have a wish and a hope that you’re going to get what you want and need. The money you spend and the time you invest with a developer documenting requirements is your insurance to getting what you want with a dependable estimate of what it’s going to cost. Furthermore, just as in the example of the contractor, you have a plan, which if you choose, you can present to other developers for bid. Beware of those who would quote you a price without doing requirements.