I know Robert brought some money but I didn't think it was 'big' money like Lance Stroll was providing??
From what I gather, it's about 50% more than any other pay driver willing to take the seat was offering, given that the previous high bidder's backer got arrested. It's a big enough amount to mean the difference between Williams making a profit and a significant loss, which means he's paying for next year's improvements to the car. Which he can't do if Williams needs to refund his sponsors.
I don't think Kubica has failed; remember his car still hasn't been fully repaired since the Australian GP qualifying (where it crashed due to working properly for the first time all season), and the damage appears to have been a factor in Robert's qualifying crash in Baku. Next race, when I believe both Kubica and Russell are due to get new improved chassis, we can start doing reasonable comparisons.