I would use tools such as expert judgment, analogous, parametric, bottom-up, three-point estimate (PERT), among others. These consider factors such as
* cost of labor (analysts, designers, programmers, testers, project management, product management, etc),
* cost of materials or developing your deliverable (codes, components, API, PC, etc), work packages, etc * opportunity costs (in make-or-buy decisions) * cost of quality * cost of risks and/or opportunities (reserve analysis)
* cost of software life cycle (including maintenance cost) * vendor bid analysis - you may ask some people to quote for you. * list is endless, but I would take the above as important.