The reward seems to be copied to each CMDR the mission is shared with, so for instance if you just do it yourself, you might only get +, with each additional CMDR adding their own reward's +. Conjecture of course, but it seems well founded.
It's a little hard to guess if the missions, done solo, will be scaled for their time to complete at all. Regular missions mostly are - e.g. sourcing missions providing ++ compared to delivery's +. Passenger missions though don't follow that format, instead according rep by the same measure, with less influence handed out for e.g. tourism round trips than for simple economy stops.