This ties in with the propositions process and estimating for projects. The idea is that you should be able to do a quick estimate to get a ballpark figure to work with so that the project has a good idea of the magnitude of the work. This can feed back into the priorisation and cost-benefit analysis, plus budgeting. However, this estimate should not be used for detailed planning, as its accuracy is only +/-50%, as there is limited information available to base the estimate on. You don't want to spend £50,000 on people's time to perform a detailed breakdown of how long a project will take. It won't be accurate, and you've already spend £50k that could have been spent on building the thing. Once everything is approved then you should enter a second phase of more detailed estimation (Iteration, Story, Task) until you have a task by task breakdown. This should be estimated in Story Points, and the true cost of the project will only become apparent as you monitor your Velocity and translate Story Points into mandays. This is hard to explain to a project manager, but if you provide them with the statistics and accurate predictions as the project team settles down, they will appreciate the benefits that this approach gives them.
The Wicks Kitchen
Nigel likens this process to the one he experienced at Wicks recently when ordering a kitchen.