The interior design fees are very different and vary enormously depending on which area they are used. When you use a professional interior designer it might be quite costly as some of them charge already for the first meeting where you are only deciding to hire him/her or not. All designers do not only charge for the products they use in changing the design, but also their fee. The more experience the designer has, the higher are usually their prices, but also the quality of work is better. Therefore, do some serious research about different interior designers and their prices, but also try to find out something about their previous works and the relationships they had with their previous clients and how satisfactory was his/her work. It might sound like a lot of work, but eventually will save you a lot of money and time, as the most research you do the best overall result you will probably get in the end.
When you decide to do all of the changes in your interior design by yourself, the fee of all the work is only the materials that you use. Although it might be cheaper, but on the other hand if you have not done it before the result might not be as good as you hope.
So think about what is the maximum fee that you could pay for your interior design and decide how you will have it done.