Summary
Although using math's is more complicated than simply guessing a price for quotes and estimates, the extra effort produces a result that is more informed and therefore more likely to be accurate. This more informed result in turn reduces the risk of your projects resulting in lost profitability or even an outright loss—this being where your costs to deliver the project is more than the client is paying you.
The less complicated type of project can in general be estimated as a standard package price—e.g. a 5 page website with bespoke design and conversion to web code to be X hundred pounds, and then Y pounds for each additional page added. This is a reasonable way to approach projects that are low risk in terms of complexity and where you are competing in a price competitive market. Using a client rating based on your knowledge of previous dealings with the client, or of other clients who show similar characteristics to the client in question, allows you to adjust any simple estimates...