I know that this is a little late but there has been a rise in lumber costs. I went through a lot of builders before I decided to undertake an owner build. You will also notice a price difference depending on the supplier that they go with as well. Some builders get more of a volume discount due to the amount of materials that they purchase through vendors as well.
Overall though, there has been a bit of a rise in material costs over the last few months. If you doubt the information that the builder gives you, call a few of the suppliers to get an accurate inflation rate as they will know exactly what is happening.