"When are new home builds most affordable?"
 As a residential planner, I get asked this question alot.  There are several variables to consider, beyond what any builder has control over.  These two factors are:  Market changes in cost of materials and interest rates.  When these two are favorable, you have the lowest cost to build.  

So, where are these right now?  Interest rates are still very low, currently today at approx. 4.75%.  If you look at the graph below, the interest rate chart shows that today's interest rates are better than any of our parents saw throughout their lives.
 
Cost of materials: Materials prices, historically, are lowest in January and February.  This is because demand for building items goes down in the winter season in our colder climates.  As we approach Spring, materials suppliers anticipate building cycles and price materials higher as demand increases.   

Currently, the best way to get the best pricing on a new home build, is to plan ahead now, facilitate your builder to pre-buy volatile materials, and use a mortgage provider that can lock in low interest rates up to 90 days prior to the conclusion of your build.