Create a formula to estimate cost of project

I need to quote for projects and i am looking to develop an estimation formula in excel along with charts. I would like to be able to estimate how much a project will cost based on a number of factors that i will have at the start of a project. However i only have data based on 10 previous project to develop a formula to create this estimate. I will also need an explanation as to why this formula will work for future projects.