I took a look at the Dominican Summer League for 2015, and spotted these significant future major leaguers among the league leaders, then looked up their signing bonuses:
Keibert Ruiz: Dodgers, $140,000
William Contreras: Atlanta, $10,000
Johan Duran: Arizona, $65,000
There were additional future major leaguers, either ones who have already washed out or who have bounced around (including two who eventually found their way to the Twins, Diego Castillo and Vidal Brujan).
My interim assessment is that there certainly were some gems to be found, but it involved a whale of a lot of sifting to find them among the dozens and dozens signed that year, and that the bonus outlay doesn't strike me as highly correlated to success.
My scanning wasn't systematic and I could have missed something important. Someone could redo it, perhaps some other methodology, and reach a very different conclusion. I also don't know where to get a complete list of 2015 international signings to see how those investments went - maybe those players didn't go to the DSL.