Going to be worth it
In the long term, the benefit to human civilization of an improved Yarkovsky effect model will be worth a heck of a lot more than the $800 million price tag (Note that this is completely aside from all the other scientific gains that could be realized from the mission). It will enable more accurate long-term prediction/tracking of potentially dangerous asteroids, allowing astronomers to focus their efforts on actual threats and potentially carry out diversion missions as early as possible. Early is cheaper, as anyone who has ever sent out a Kerbal team to shift a Class E rock surely knows.