My wife looked up the rate from our main bank [Affinity credit union] for 30 year fixed for 500K last night. It was about 6.875% (60 day lock-in) with about $3700 fees or 6.910% APR. As you mentioned, we may not get this rate, but at least we could know the ball park. We used this bank once before and we may use this bank again if we cannot find the better one.
As you mentioned again, it may not be wise to use builder’s mortgage company, but we will check.