This info is summarized in this Wikipedia article: https://en.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_He...
One booster seems to have been reused in just 2 and a half months (launched July 25, 2018 and October 8, 2018), compared to a new-production time of about 18-24 months from aluminum stock to launch. So a ~tenth the time of production and so probably about a tenth the labor (for the first stage).
As far as slowing down production...
...SpaceX's production rate of new boosters has been roughly flat while their launch rate has increased, so this effect has not occurred. In fact, you can make a good argument that the primary benefit of reuse for SpaceX is that it has allowed them to squeeze a lot more launches (double) out of the same production capacity. They launched cores 23 times (3 at once for Falcon Heavy) in 2018, and only 10 of those were new. In 2017, it was 18 total launches, 5 with reused cores and 13 new.
SpaceX can switch resources between upper stages and first stages, too, and eventually mothball first stage production or convert it to BFR production, thus giving them a benefit from reuse that you might not consider from a first-pass analysis.
Only if you assume the market demand is constant. We already know that's not true.
They still have to build expendable second stages for the falcon 9 and that uses practically the same engine (it's optimized for vacuums). Also, I doubt rocket companies gain much from economies of scale considering how low launch rates are so scaling back production probably doesn't increase costs much.