For those of you unfamiliar with "The Mountain Witch," my game uses a "Trust" mechanic involves spending points for one of three purposes:
1) aiding another character's rolls
2) Narrating other character's success/ failure or
3) Influence other characters to do something (in other words, *force* the character to do something).

I have some issues with the third option, however. In my original version, I basically said the owner of the influenced-character could set the price to whatever they wanted, according to a vague standard of the "impact" of the act. Mike Holmes commented that allowing the influenced-player to set the price might cause the economy of the system to break down.

In my current version, I set definite prices according to whether or not the influenced character would agree with the action. While not as problematic as the original version, it still allows the influenced-player to set the price.

I wonder if this is the best way to set the price for influencing. I like how the current system allows for a degree of freedom, since different actions obviously carry with them different risks. However, in my playtesting I've seen how allowing the influenced-player to set the price can sometimes be problematic. [In one of my playtests, one character told another to attack a group of orges from the front (alone), while he and the others would sneak around and attack from the rear. Personally, I didn't think it was a big deal, but the owning-player thought it was a horrible idea. He ended up charging a moderate price, but I could tell he was considering a high price.]

A recent idea I had was to base the price on whether or not the action involved entering into a conflict.

Does anyone have opinions or suggestions on a way to structure pricing?

PS: If you would like to check out the actual game text, follow the link in my sig.

Seems to me the mechanic has a self-moderating feel already. Otherwise, the player in your example would have set a high price rather than a moderate price. The same metagame factors that are in place to limit what you'll influence another player's character to do are also in place to temper the influenced character's player's price. In short, I don't see a problem based on my read of the game. I think that it will work unless there is serious disfunction happening in the play group, in which case there's nothing you as a designer can do.

Another option (only available if there are 3 or more players) would be to allow the player(s) not directly involved in the trust transaction to set the price.

So if player A was using trust to control player B's character, player C would set the price. Or, if there were 5 players, players C, D, and E would each write down a price and the final price would be the average of their 'votes.'

The downside (and upside) of this approach is that the cost determination is taken out of the hands of the players who care most about the action.