Do you really think Donskoy would normally lose to Evans, I think not, Donskoy might not have gotten one, I still think maybe he did, he is big cause he's a top 100 player, who don't normally play CH's, but certainly they were going to give one to Kubot as he was a WC and Wimby QF, Pospisil might have one too.
You would be surprised what some CH's can do, some have good money.
I watched the Davis Cup match between the two in which Evans absolutely killed the Russian from the baseline, so, yes, I gave Evans a good chance here. The question was more whether Dan's head would be in it, because the match-up (and Donskoy's form) favored him.
Losing in three sets is also not an obvious tank.
I am sure sometimes appearance fees happen, e.g. in tournaments like Prostejov. But then the money goes to big names (i.e. not Donskoy) and those players receive WCs. I highly doubt that the USTA promised Donskoy appearance money many weeks ago when the entry list came out; they would have held on to their money in the hope of luring a real star.
Even in this field it would make more sense to give appearance money to players like Rochus and Becker, who achieved much more in their careers than Donskoy.
Just look at the Russian's results. He needed to be here. I didn't watch the match, but I would be very surprised if Donskoy just threw it.