How do you value a real-estate development project?

I would like to hear how other value their real-estate development projects (assuming that we have any developers - or aspiring developers - here?). To kick things off, I am attaching a spreadsheet that I recently used to 'justify' the purchase of a $1.3 mill piece of land, and to 'estimate' the total cost of the project (about $18 million to build).

[BTW: for those who are interested, I will be running through this project in a series of posts on my blog, starting with this one: http://7million7years.com/2009/09/09/ive-been-out-shopping/]