My party is around 30-31 level and training cost is big problem for me.
It even make me hesitate about promote class to second rank, just because of training cost.
Is this temporary problem for slightly low level? or all Master Merchant is not a bad decision for rest of game?

I gave all Master Merchant (and Master Scholar) pretty quick and never had problems with money.

30 is pretty early, I finished the game at around 120. So if you are having problems now, I'd say get Master Merchant ASAP.

Just out of curiosity, how much does it cost now?

120...wow. I must train merchant to Master ASAP. Thanks! :)

And training cost was around 1000 for 7 rank master merchant and 1500~1600 for 1 rank merchant.
single cost was not that bad, but cumulative costs make me sick.

Same here, about level 120 when I was done. Might and Magic VI is one of those good old RPG where devs didn't care much about stuff like balancing, unlike Might and Magic VII, as such there is a totally legitimate, non bug exploiting method to acquire infinite gold in the late game. Merchant skill do not affect the price of training, only the price of buying and selling items if I recall correctly.

In my playthrough I gave the merchant skill to only one person for 85% of the game. Typically in RPGs where item management is a problem, one of my character carries the quest items, wands and potions, another carries the magical armors, another the magical weapons, and the first guy in the party carries the stuff I want to sell (or buy) to the shop. If I recall correctly, late in the game when I was finalizing my equipment, getting rid of wands, weapons and other useless items, and doing the trick to acquire infinite gold, I gave my second fighter the Merchant skill too so I would no longer have to move the items from my second fighter to my first character.

In MM6 at least it definitely does. If I'm not mistaken, in 7 too.

I stand corrected.

Has anyone ever found a "Wandering Trainer." I couldn't remember if Trainers were effected by Merchant Skill or not so I looked it up in the Manual. Though it didn't say anything about cost, there was a mention of "Wandering Trainers." I don't think I have ever seen one. Has anyone else?
(Edit) Re: M&M VI

If you read the readme file, under the manual errata section it was mentioned that this was removed from the game.

Page 36. There is no Join button. Click on a NPC to initiate a
conversation. Select the Join topic to have the NPC join your
party. You may have to Beg, Threaten or Bribe certain NPCs to
get them to join your party. There are no Wandering Trainers
in the game.

Now I know why I never read the Manual. LOL. Thanks.