It isn't just another launch, every future launch with this same rocket is now an experiment. It can't have the same reliability rating because a second launch hasn't been tested and these aren't freshly machined parts. Every piece in the rocket has experienced a level of wear and tear that previously launched rockets didn't have. Maybe it's exceeded the amount of cyclical stress it can handle? Who knows, but most satellite companies wouldn't pay the same for a re-used (never before been re-used) rocket as they would for a brand new rocket because of the increased risk. I would not be surprised if they did in fact give a discount to the payload of anyone who rides this same rocket. If they don't give a discount the only reason someone would purchase a spot on it is for the prestige.

Note: The never before been re-used part is probably the most important factor here. Once they have re-used rockets a few times they'll have a reliability factor for these situations.

At this point, they have the functional monopoly advantage of being able to set prices where they want them (in terms of having an exclusive technology their competitors do not have; they have competition, but if they can really slash their costs by 50% they have the capacity to set their price ten bucks below the competitions' prices and win).

... but if I understand correctly, one of the points of the reusable rocketry experiment is to be able to lower the price point to allow new customers who couldn't afford to launch at the previous per-pound cost. To give a terrible but not inaccurate analogy: Think similarly to how the Nintendo Wii found itself in entirely new markets of game players by droppping a $200 console onto the market alongside competition that was nearly double that price-point.

Opening up space to new markets must be an essential component of their strategy. If they charged say 90% of the cost for non-reusable launches but their rockets were reusable, they would only need to manufacture a dozen or so vehicles, plus a few a year as replacements. To maintain economies if scale in their factory, they need to have a sustained production line and that means at even a steady pace, with high re usability they will soon be awash with rockets. They will have to bring prices down just to keep their fleet operating.