What does the support footprint look like? Does it include a telephony component? Live chat? All these factors greatly modify the support cost and can get out of hand very quick. I would suggest scoping out what the best in class service looks like from the corporate stakeholders perspective and then start focusing on costs both in terms of labor, software and hardware. Then start backing out components and specific requirements. For instance you may want 24x7 coverage but do your really need it. How about language support are you launching in multiple countries that require mother tongue speakers? If so, the added cost there is not small.