I used to solo avatars daily on RDM/NIN (6 of the 8) and have done them on my SMN75 as well...
There is only one real problem with avatars for gil. Time. As a non-Summoner job, it takes about 6 hours to do 6, because of the running around.
On Summoner, fights take 18-28 minutes each and are VERY risky, and its hard to control hate.
Basically though you can make 60k a day, which is 60k/hour mostly. However there may be better options depending on your jobs. :s
But, 60,000 x 30 = 1,800,000.