For the time it takes to return the investment, for me it's not worth it so I don't use it at all. If you want to make a sound investment, produce leather and sell it on the troll market for 16-17 gold..perhaps 14 at times if you're strapped for some..but I wouldn't suggest, because when you sell say 1 mil leather at that price, you're loosing atleast 3 mil in gold.
Why leather?
It's the sole product you can produce without using wood, since latter in the game you may find yourself in huge need of it and you'll be left with tons of water that you wont know what to do with.
Stone is another material you'll rarely use in game if you got all upgrades, so if you really want to sell on the traveling merchant, produce a ton of stone plaques/jugs and sell them simultaneously.
If I were to sell stone plaques right now, I'd have to put 40k for a one long month investment to return 1 mil gold. I can make that in a day with two dungeons visits.