How do I look up the original build prices of homes in my neighborhood?

[Kyle Reedstrom]

I have a lot of new construction homes in my area.   Many of them have no transaction history.   How do I find out what the price was of these homes when they were built and closed with the 1st owners?   AKA I want to find out the original price the houses started at?

Context:  I am a Real Estate agent and we are conducting a marketing campaign to show people how much their homes have appreciated in value.   In order to have that number, we need the original price the home closed at.