Good modeling done prior to the commencement of actual development      will greatly reduce the overall cost of the website. Taking the time to get all the potential decision makers, in advance, to agree on the goals and process that will be used in development will give you the best chance of getting you what you want for the budget you have laid out. This preplanning is not free, of course. You should account for the time spent internally, including that by the top-level decision makers. And, you will want your web development company being involved in the preplanning process to a great extent.

Creating a website involves strategic thinking, analysis of needs and audiences, awareness of where you want it to develop over time. Having the right information architecture and good design notes on how it should look before starting will save a lot of time and money later.

The preplanning will result in creating the prototype of your future site, displaying all its elements and interconnections; functional specification with the detailed description of system functioning, interface requirements, system features,  quality assurance types required; rough estimate providing the hours spent on any activity and $$ cost with the description of each module.