At this point in the development process you will have a clear idea about what the site should be doing for your business and the people who will be making it their business to visit your site.

The jigsaw is beginning to come together.

But wait a minute; your site doesn't actually do anything yet. Now that we have the business objectives and target audience, defining the functionality should be a breeze. And it is, so much so that you could end up with a list several pages long. The list needs to be prioritised. Start by assigning a score (0 to 10) to each function on the list, based on what the function adds to the site in terms of

(i) potential visitors, and
(ii) achievement of business objectives.

To help with the process, we can give each function a notional cost based on how easy it is to build. Whether this is desirable is clearly dictated by the criticality of the function, the less critical functions may need to go through this process to determine whether they should be built or not. The easier a function is to build the less time and hence money is required.

Finally, sort the list based on your scores and the notional costs (where appropriate). Those functions that fall into the "not enough time/money before launch date" bin, can always be resurrected at a later date.