These will vary dramatically depending on the size of your start up business as well as the type of business that is being started. However, some of the costs that will need to be considered are:
How much it will cost to build an inventory of the products you will be selling.
The equipment you would need to use, for example, technical equipment.
The cost of building a website.
Marketing costs and possible outsourcing of this.
Company formation costs, if you decide to go with a limited company structure.
Domain cost, there is usually an annual fee behind this.