The cost would not increase linearly for 8 people. Dragon 2 was originally designed for 7, NASA requested it be configured for 4. If they wanted to go to 7, my guess is it would be a pretty marginal increase in cost.

The shuttle didn't have any real abort capabilities (other than very late in the flight, when they were already in space, just not in the correct orbit). It had some crazy schemes for earlier stages of flight, but they were never tested, and no one actually thought they would work.