Your interest rate would be variable, but if you use the increase in cash flow to aggressively pay down the principal balance, you could save significant interest cost and maintain the liquidity of the open-ended line of credit.

Too many variables E.g. location, plan complexity, quality, exterior, lot prep, etc. 

So many other variables that are not being addressed in your question, the biggest being price difference which can cover a lot of sins.

Many HELOCs I've seen have low intro rates for a year or so and then they become variable, so you also need to consider the fact that the interest rate for the HELOC could rise.  3.

I recommend you do this each year to adjust for utility increases and other variables.

There are a lot of variables to what you are asking. 

Just an aggregate of the sales data for each neighborhood, which is still subject to the variabilities you describe, but I was just looking to find some neighborhood trends.

It was only years later that understanding the variable rate, that I knew I left money on the table. 

I recommend you do this each year to adjust for utility increases and other variables.

I am not an attorney and don't want to provide my thinking on this, there are a lot of variables that factor into this.