What is an efficient way to make gold?

Based on the great answer by @lentient, I'll expand on my specific method for farming gold. What I do:

• Buy 99 gold rods from Gormott (500k @ max rep) - if you can't buy 99, even 10 is a net positive gain. I bootstrapped this whole endeavor with 30 silver rods, which cost <50k, so that can be a place to start, too.
• Head to the Saets Chief's Residence salvage point
• Salvage 33 rods at a time, then sell

Selling specifics:

• Start from the bottom and go up. Anything >4k is worth exchanging for.
• Sell Morph Orbs by the 50. That's 100k, and you're going a disproportionate amount of them.

This whole process takes a max of 1.5h or so. You should make somewhere in the range of 2-2.5m by the time you're done.

To think about it, there are 4 ways to earn gold - farming enemies, farming collectibles, looking for treasures (hidden chests on locations) and salvaging.

Farming enemies is least favorable, as enemies of same level as character take some time to be defeated, drop mediocre amount of gold, and may provide some good sources of gold only as sellable items like trinkets and aux cores.

Farming collectibles may be safer and faster way to gain loot to sell, but prices are kind of questionable (given most expensive item I have costs 110 gold per piece, and I have 14 of those by start of chapter 4).

Treasures may yield nice loot and noticable amount of gold, but have serious yet obvious con - they are exhaustible.

So, actual answer - my bet is on salvaging. Even basic cylinders will yield you some nice items for properly performed "entry".

As example, I've got 10 Normal Cylinders in Torigoth (not sure what price, I had Development lvl 2), and spent them on salvage point in center of city, which is relatively low-level. All runs yielded me ~20 basic sets of Torigoth, Strong Snow Spring, which is 6000 gold. And it is not counting that each chest gets you 1-3 actual treasures, 200 gold each, and some possibility to combine those to more expensive sets. And last, but not least - selling treasure sets actually gains you Dev points! By selling sets, I've got Dev lvl 3, therefore now I can buy new cylinders even cheaper, which increases total net gain. Also, as I've specified above, I was using low-level salvaging point, more decent points may grant even more profit with same cylinders.

PS. I've described only ways to earn gold on relatively early stages of game, as got myself only to chapter 4. There might be new mechanics to gain gold in further chapters.