Your initial investment will depend on the type of business you are going into. Some businesses require a significant investment in equipment or technology. Some businesses will have significant overhead expenses such as insurance, phones, supplies, advertising.
The other thing to consider is the decrease in income you will have when you start your business. An ideal business would have minimal capital investment, low overhead, and you would be able to start while still employed at a regular job. New businesses tend to underestimate their start up costs, while overestimating their sales potential. This cash flow crunch is the cause for a majority of new business failures.