How's this for an answer; The reason that YNB and not YPD is preferred
is that you want the cells to quickly suck up the FOA. Although YNB
does have plenty of nitrogen, there is no question that cells grow
better on YPD and therefore may not take up the FOA as rapidly.
Therefore you need more FOA which in turn requires you to ultimately
spend more money! There was a paper put out a few years ago showing
that, in fact, if you reduce the amount of added uracil (which allows
mutants which will be resistant to FOA to survive) you can use less FOA.
I have found that if you do that as well as reduce the level of dextrose
you get even more mutants--most likely due to starvation which causes
the cells to really crank up the permease activity.