I'd like to ask the opposite question: How much money do you have? Getting this answer may be easier than you think. If you let me explain a few options.
Very few SharePoint installations with much success are simple and straight forward. Microsoft should have marketed SharePoint more as a platform than as a product. In that respect, answering what a SharePoint installation costs is about as simple as answering what a .Net application costs. And while you may probably be able to deduct some kind of estimate based on several input parameters you are more likely to forget something or change the scope when you start working more on the specifications.
That being said, there are several things you can do in order to control costs, which might help you along the way. Agile based development methodologies works on the premise that everything is more or less changing. Short development cycles, called iterations, deliver production ready features for every iteration, with each iteration having a pre-determined cost. Thus you focus on the primary deal-breaker functionality first so you have something you can use, and then you add features as long as you want to spend money. While this is a greatly simplified explanation the point is that you control your costs by adding features with fixed costs and when you stop you have something that can be put into production right away. During the development lifetime you will likely focus on the features that gets you the highest return first. The idea of Pareto diagrams is greatly beneficial in this process.
Let me take an example, a business process that costs the company $20,000 a year. Workflow-wise, costs are again difficult to predict. Using an agile approach you may define sub-features of a business process. So you can start by defining what the basics are, perhaps costing $20,000. Then you find that it reduces your cost for that business process 70% but you need better exception handling (non-intuitive forms, too many people get involved if certain things happen, etc).
You improve the workflow for another $20,000 yielding a further 20% cost reduction. Having achived 90% of a 'perfect' you spend another $10,000 on improving performance, giving you another 2% of the way. Reducing the cost further may be too expensive compared to the cost. You end up at 92% of perfect and have controlled your costs all the way. Even with 'only' a ~92% reduction in cost, you still have a pretty good return on investment over five years, with your development costs at $50,000 and business process cost still at $8,000 (8% of $100,000), totaling $58,000 versus the 'unoptimized' cost of $100,000.
In conclusion, getting a reliable estimate before any thorough analysis is close to impossible. You will likely find that controlling cost may be a better option. Your first order of business thus should be to get a budget by estimating costs reduction or profit increase. Estimate how fast a return-on-investment you need and then start looking at how you can get your optimalization going and what features of that optimization gives the greates return.
Oh, and by the way, if you find the numbers too big or too small for your project, just remove or add a few zeroes at the end before presenting to your boss :-)
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