
Wathabaska |

I suppose it was an oversight; the best fix is to either make the iron balls ceramic or stone... or to make them magic. The builders of the tomb were elemental creatures; perhaps the iron balls are conjured from the elemental plane of earth, and if they're taken from the cairn they rust away.
Personally, I don't have a problem with the adventure as written; if the PCs want to harvest iron from the trap and "get rich" that's fine. They should find that the market bottoms out quick, and it should be a LOT of work not only to haul the iron out but to fight off claim jumpers and bandits (who might also be well-connected mine managers like Smenk).
It's a little unfortunate that there are certain things that the D&D economny won't let you easilly do in an adventure. Other examples are to have a low-level adventure where the PCs defeat a boatload of pirates (suddenly they own a ship that vastly exceeds the gp value they should have access to); encounter a bubbling lake of poison in which swim demons who can splash the poison on intruders (how many vials of poison is a lake, and how much is all that poison worth?); or defeat a group of goblins that have holed up in a mine that no one owns (how much is it worth?).
I also saw no problem with it. If the players want to mine the trap, then great. They will need to hire laborers to remove the iron, since there is way too much for them to do by themselves, in any timely manner. Diamond lake doesn't exactly have a lot of unemployed miners, so unless the players want to poach from the mine owners by paying premium they will have to import labor. They will need to hire teamsters to take it to market, as there is no market for iron in diamond lake. These employees will cost money, lowering their profit.
Then they will either have to purchase the cairn. I'm sure somebody owns the land and is likely to show up quick when it becomes apparent that there is something valuable there. If the land is unowned, then I'm sure Mayor Neff will show up with the proper paperwork showing his ownership pretty quick.
Assuming that they do all this then they sure can make money, but the curruption that is omnipresent in diamond lake will eat up a lot more of the bottom line.(a significant amount of their workforce will probably be on the payroll of Neff or Smenk) Also the money is not going to just show up immediately, it will filter in over time. A slow steady income is not going to make that big a difference. Especially with all the money they will have to layout to get the operation started.
A better option, if the players were real clever, would be to secure the rights to the cairn quietly, before anyone knows that it is valuable, then sell the rights to a developer. Again this probably wouldn't get them too much to ruin everything.