You will catch some grief on this topic, but I will start off by pointing out that no one can accurately tell you how to estimate this....estimating is a difficult trade to just jump into, since most estimators have been in the field long before they jump into the numbers side.
I know a guy that used to clear retail properties, which is not as much demo as you would think...he basically gave a bid to empty the area, and salvaged the fixtures for his profit.
You can try it on the "safe side" by seeing if your customer will agree to a cost plus basis......hire your labor from a temp agency so you are covered on WC and insurance, and then be on site to run the show. At the end, you add up the costs, add 10% (or whatever you agree to) and bill the customer. Take the percentage as being paid to see what it costs, and then next time, you will know how many man hours are involved in gutting a retail space. Good luck.
BTW, in the contracting business, demolition generally refers to tearing down of existing structures, not what you are doing.