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It sounds like you should probably retain a CPA to perform due diligence work on your behalf. You can compare expenses to industry averages, but you can never be sure whether the data you are given are accurate without checking for yourself. Since you already own a dry cleaning operation, you already have an idea what it should cost to deliver the service. My guess is that you think your target's costs are too low and I think you are probably correct.
If you select a practitioner that also has business valuation experience, then he or she would also be able to suggest a purchase price after the work is done.