> Would it still worth the huge capital expenditure to develop space based resource mining/extraction to reduce the amount of mass that needs to go up form Earth?
Yes, because asteroid mining plus self-bootstrapping manufacturing systems leverages the launched mass by hundreds to one. Bootstrapping means sending core machines, grabbing some metallic asteroids, and machining them into parts for more machines, like chemical processing units. Keep doing things like that till you have a whole factory. Asteroid mining has a mass return ratio of about 200:1 on the mass of the mining tug. If the core machines are the same mass as the tug (30 tons), your net return is then 100:1.
Also, a BFR, or any rocket, doesn't fly efficiently unless you are launching at least 6-20 times a year. There currently isn't 600-2000 tons of annual launch traffic, and certainly not in 100 ton loads. You only need a BFR if you are doing something like colonizing Mars, in which case the 100:1 leverage is very useful, because propellant to get to Mars and stuff they need once they get there is a lot more traffic than we have today.
Also, if it's fully reusable, and only flying 20 times a year, your aren't building many new BFRs each year, so your production line for them isn't very efficient and cheap. Space mining can reduce the size of your launcher and optimize the launch and production rate so the whole system is running at optimum cost and efficiency