As to whether Jack was "fleeced" by Marvel, apart from the art return issue, I think there is a strong argument that he was. I can't cite sources, but I've read several accounts, first and second person, stating Jack was promised a percentage of licensing for, for example, animation projects, and general verbal commitments that Jack would share in the success of the properties, and these promises were reneged upon by Martin Goodman, to Ditko as well, leading to the departure of Ditko in '66 and Kirby in '70.
Moreover, Jack was from the beginning paid only for pencilling or layouts of the books he appears to have created from whole cloth, while Stan was paid the full scripting fee for books that Jack plotted (by Stan's own admission) and Stan wrote the finished copy for. By any rational definition, Jack did part of the writing of those books and was never paid for that work.
Finally, there is Marvel's culpability in the falsification of the historical record regarding the creation of the core characters and settings of the Marvel universe. The world regards Stan as the genius behind this achievement, despite the fact that all the evidence points to Jack for the lion's share of the work. This deception certainly cost Jack money, though no one can say how much.