i've noticed that a lot of people are interested in merc services from other players, yet will not take them because they are afraid of being scammed. hence, they usually will not pay for a merc service before the service is done.
mercs on the other hand, also do not like being scammed. hence accepting to be paid only after the service is done is not a good option for mercs either
so i say: formalize it! make it so neither player can be scammed.
make a new "merc interface" window. that way the merc can set his price. note: this is essential. this system would be ruined with arbitrary price-fixing set by the game itself.
anyone joining the merc's party will have the agreed-upon fee paid to the merc automatically. or the merc can vary his price for each player. or waive the fee for players who he does not want to charge.
have the amount of money that is charged to each player be logged, and have that money tied to the variable condition that all the players in that party win the mission that the merc has charged them for
if the merc leaves the party for any reason, then all money is refunded automatically to the people who paid him, in the exact same amount that they paid
if the merc and his party loses the mission, then anyone who leaves the party after that gets his money back automatically. or if everyone stays in the party, the merc can try again by restarting the mission
if the merc and his party wins the mission, then the merc keeps all the money he has been paid and the deal is over
N

(in which case one person could not do it alone anyways)).