That time/material difference you are seeing inside of FLUX Studio between the three different embedded slicers is not super accurate and is very, very model specific. Try it on several different models and you will see that there is no 'standard deviation'.
Also, we should all probably clarify a bit more when we are talking about the different slicers whether we are talking about using the embedded versions or the standalone versions (which have more features) and then importing g-code.
The new Prusa Slic3r is very cool with the variable layer height options, it really does dramatically speed up print times and save filament while still giving detail where needed. It isn't for every print, but when it fits it's awesome. The same can be said for Cura 2.5 standalone, it's got an impressive set of features once you learn how to unhide everything and see all the options.
Using either of those two slicers combined with Meshmixer or Craftware if you need to add custom supports and it would be hard to justify $150 for S3D anymore. I do still use S3D a lot, but that is more of a production/convenience factor. After playing with the other new features though, now I mix it up more often again. I am curious to see if S3D is going to release a big massive, amazing, long overdue update, otherwise, @Tiwaz is absolutely correct.