They may be standardized (available in published texts or purchased commercially) or proprietary, depending on the type of business, product, or project in question.
Models typically function through the input of parameters that describe the attributes of the product or project in question, and possibly physical resource requirements.
The model then provides as output various resources requirements in cost and time.
Some models concentrate only on estimating project costs (often a single monetary value).
Little attention has been given to the development of models for estimating the amount of resources needed for the different elements that comprise a project.