It really depends on which interior design firm you choose and what you’re planning to have done. When I was looking to have my restaurant redesigned, I met with a selection of restaurant interior design firms and their prices varied a lot.
If you want to get a more accurate idea about prices, the best thing to do is to contact firms and discuss your project with them. If you want a reliable interior designer, it’s likely to cost you a reasonable amount and you should be wary of any prices that seem too low. In the same way, some firms will try to charge you a lot more than the going rate, so it’s a good idea to guard against this too.