I'm looking at offering clients an in-house service or spending money and outsourcing the ability to offer clients an e-mail newsletter service. One of the major determinations of cost is the size of the mailing lists. I expect my clients to start with an empty list, but I still need figure out how much it will cost me. I'm at quite a loss as to how best estimate anything reasonable. Can anybody give me anything to work with or any kind of hint?