Create a calc that does something like this:
IF <date of this row is in the range I want to look at> THEN [Price] END
You can figure out what you need to put inside the brackets.
When you set a value after the "THEN" clause without doing an "ELSE", all other values are set to null for you. So your calc will contain values only within the needed range. Use that calc to do what you need to do.
It sounds like DATEDIFF is what you need.
DATEDIFF('day', [Go-Live Date], [Sale Date])