Tips on making money
first thing to do is familiarize yourself with potential drops and the monsters you fight against in the various BCNMs, the above link points toward the official BCNM FAQ
overall the drops from all the level 40 cap BCNM40s are the same, this is because the two spells Erase and Phalanx are both available in all BCNM40, they are the big money makers (worth 100-400k each depending on server)
people choose to fight the BCNM40 in Giddeus against the Mandragoras because they are by far the easiest of the three choices with your average PT lineup (though you need a Bard, no exceptions)
the other choices, Worms or Rabbits, have a larger number of monsters to contend with and are generally not done at all unless you have at least 4 or more beastmasters to tame the enemies and have them fight for you
in this sense the quickest and easiest BCNM40 is probably the Rabbits in Horlais Peak (Ghelsba/Yughott) done by 6 beastmasters, the fight is over in 6-8 minutes and there is very little risk, there is also very little travel time between fights
for any normal BCNM40 group the Mandragoras are the easiest and therefore the best choice
the question of drop rate though is a completely different matter... personally I am of the opinion that shorter fight times equate better dropped items which is my overall impression from something like 40 or 50 BCNM40 fights