Every Interior Designer has to learn how to price their services. You have to do this so that you can give proper estimates to your clients. This will be invaluable to you as a service provider.
Many people find this to be a very difficult process because they don’t want to over price their clients, nor do they want to lose money on the deal. This can be very frustrating and stressful. However, if you know how to break down the costs it gets a lot easier, and helps you break it down to the client so that they are comfortable.
Since every job is different, every invoice will be different as well. There are different types of invoices that you should have. They each require a different layout and outline. This makes it easier to break down later for calculating the costs.

This type of invoice is for designers that have to hire outside contractors to help with the work. (usually when you need to hire people to do drywall for construction work etc)

This type of invoice is generally just for the costs of the extra furniture and accessories or lighting fixtures that are needed. It also covers the agreed upon extras like paint and wallpaper.

This is the type of invoice that only covers your prices as the designer. It deals with the work that you do alone. This invoice is always included with one of the others.
When trying to decide how to put a value to your services alone, that is pretty much up to you. It is best not to price yourself too high in the beginning because you are not a bankable name yet. There are two ways to charge your clients.

This method is best for small or simple projects because they are more difficult to price on a per project basis. If you are a starter, you should start yourself at around $15-$20 an hour. At least until you establish yourself in the field. Once you are established you may charge as much as you’d like.

This is generally reserved for projects of a larger scale. These are the projects that require the hiring of contractors and various others to help you. Since there is so much work involved, you cannot expect to charge by the hour.