@Qiller
The comparison to fighting is pretty simple actually: you lost the goods that 'died' in the negotiation game. You guessed wrong and lost what was on the table.
As for the battles themselves we don't get 3 tries at a fight for free either. We can retreat but once it's the enemy's turn we take our lumps and leave the field with what's left if we don't like the results. Same as the negotiation game.
As for some of the other issues:
Attached troops do not cost just coin/supplies. They require barracks which require free population and space in our cities. Build more goods factories if you want to guess more just as we have to build more barracks if we want to keep fighting as our units take damage and/or are killed. yes the Alcatraz great building can generate free troops but that requires quite a bit of space in our City and quite a few forge points to get them up to generating sufficient unattached troops. In addition to be able to win we've also dedicated space in our cities for the attack great buildings and the required number of forge points to get them our attack and defense boosts to useful levels. There is actually a Great Building that you can use to generate 'free goods' every day: the Dynamic Tower. Probably won't be enough by itself to pay for the negotiation game but neither is the traz by itself sufficient to win thru by attacking.
Please remember that the primary bonus of this game is meant to benefit the guild and not the individual. So you need to factor in the gain for the guild before you say it costs too much.
The game itself can be done without spending too many diamonds. I've negotiated a few times (and at the higher levels) and gotten it done in 3 to 5 turns. There is a strategy to narrowing down the options that if you employ it that can help you solve the encounters quicker. If you don't like how the game has gone because you haven't narrowed the options down enough then retreat and try again.