If the client is asking for an implementation as per your example, you already have a major problem (and are in a race to the bottom as a supplier). The spec sh

If the client is asking for an implementation as per your example, you already have a major problem (and are in a race to the bottom as a supplier). The spec should be expressed purely in terms of business outcome (e.g. I need a system that can handle more transactions within the next 3 months). (Not to mention that the most interesting and valuable requirements are usually the ones that the client doesn't yet know about.) Second problem with this scenario is that if you are charging a fixed amount for a complete system, you are assuming all the risk (and essentially selling a form of product). Generally that's a lousy deal for a client on build cost and maintenance of a bespoke system. Few desired business outcomes are so unique that they require a bespoke solution - so this should really be a very niche business, not the norm at all.