I disagree. I think the variability in the amount of darkness in the random sub-dungeons already has more of an effect on the game than that in the smuggler's den, and that doesn't seem to be a problem.
Anyway, wouldn't the basis for balancing a preparation be comparing it to other preparations which fit the same slot (rather than trying to come up with a "gold cost" basis for it)? That's pretty difficult to do with the Thieves' guild.
You can't cut back on kingdom funding. You will regret this!