Study Bones / Ooze / Tail are the three 80gil items which take 36 to equal a 3x multiplier. So, 2880 gil gets you 3x. If you're a Gil pincher - save money and wait until you can upgrade an item to max level in one shot - save the investment to a one time shot at multiplier. Furthermore, you won't need to level up any weapons until you're at a point you should be able to afford the expense.

If you would rather just play and level up - do that - don't worry about the gil it costs you - just have fun and be happy.

While Fluids and Oils may give more EXP, they're a waste since the inorganic items are so much better - you are better offf getting to 3x as soon as possible, and then switching to inorganic.

For farmable scenes (like chapter 11) - nothing beats the bomb parts. I had nearly a two full stacks of bomb things from Marks and used them to level up most of the accessories required so I could unlock 'em all for the treasure hunter achievement/trophy.