stick to your guns, i had a business call me for a quote and the first thing they said after i mentioned the pricing structure was 'oh, you cant go any lower than that can you, as we have had several other cheaper quotes' i stuck to my guns and they called back for a booking. maybe sometimes they are trying to call your bluff, but there is is companies around that are quoting cheaper and cheaper i have found. which is alarming for sure.
tell the potential clients that your insurance is up to date, you have offline data backup, vehicle expenses, machine calibrations etc, and to check whether the other companies with the cheap quotes are up to date with the same.
i need to do around 100 tests a day for the company to make any money, if we keep lowering the prices than i have to do more tests daily which isnt actually achievable at most work sites, especially when you have to seek the appliances at most clients sites rather than having them piled up ready to test.