"Alright an additional small room will cost 1500DP and for a regular basic iron mine environment it'll cost 8500DP, 10,000 in total." Even if he had a medium sized room or even a large sized room the output of the iron would remain the same, just the space it covered increased. The mine environment followed the same rule of thumb as the other environments, room size did not affect the price, something Dark decided to keep the same. Of course, there were limits to having it in a small room, it could only be upgraded so much before requiring more space to function. Not only to house the iron ore but also for his monsters to gain access to the mine, more iron being generated every day would require more miners. But thankfully Dark thought of this plus it was refined at the gathering when the issue was brought up by Damascus who already had his own iron mine and ran into a few problems.