It depends. An existing business should have established procedures and processes, operations, reputation, as well as a customer base which can result in a high price tag. It should also have a positive cash flow. If the business is lacking in these areas, that is problematic. Also keep in mind that an existing business may also have "baggage" which can cause problems for the future owner. Existing business typically sell at a premium.
A potential buyer should consider the return on the investment in an existing business in comparison with the return on starting a new business. A new business can start with a clean slate. While it may take less money to start a new business than it does to purchase a going concern, it takes time and can be very difficult to build the business. The new business also has no track record on which to base decisions. Significant working capital is often required to grow a new business.
The bottom line is that every business purchase situation is different and should be carefully evaluated on its own merits.