there is an another way.
Just as the above's narration , the way of selling weapons is feasible .
First you can buy some kind of weapons which has 4 blanks without an ability . (There is a place you can buy them)
Then after you had conquered the BESAID ISLAND, you will get 99 stamina tonic (ST). Every ST can add to you weapon mentioned above an ability of HP+30% .
Then sell them . After a while you can earn more than 1,000,000 .