As for the monsters vs. quality ore:
good ores will be found either
1) next to cities that have been established just because of the ore, so the mines would be reasonably safe from monsters, but there\'d be lots of miners so prices will be less, and also the transport routes will be less secure, since these cities usually are in the middle of nowhere with lots of danger around them, or
2) a good way from normal cities, so that the way to and from the mines will be somewhat safe due to the town, but nor absolutely safe, and also the mines are less used and thus there are chances that monsters occasionally enter them. Prices would be higher for ore due to the moderate danger and the smaller number of miners.
I also think that the toxic fumes and other things should be implemented. They would, in fact, be the
perfect deterrent for bots. You are mining happily for hours on end, but suddenly you hit a crack containing toxic fumes. Your health will slowly fall, increasing in speed if you don\'t get out.
Also, when digging deep, oxygen may run low, let\'s say due to bad air circulation caused by winds changing or no wind at all. You\'ll get dizzy and tired, and if you don\'t get out, you\'ll die.
Cave-ins for even more excitement: they may
- happen far from you, so you\'ll hear them and maybe the way out might be blocked, forcing you to dig for your life (though in the closed area there shouldn\'t be any more bad effects until the way out is clear again),
- happen near you, covering you in dust, which reduces your sight and health due to the dust in your lungs, so you can die if you don\'t get out, or at least take some damage
- happen above you, possibly killing you.
Cave-ins need to announce themselves by strange sounds, like rumbles, small earthquakes, whatever.
Oh yeah, heat can accumulate if many miners are in the same place, the air gets sticky and health is reduced slowly but constantly, and endurance as well. At some point, you simply can\'T mine anymore, so either you get out to refresh yourself, or stay there staring at the rock that you\'re supposed to be mining.
I think that\'d make mining more exciting, at least it might make me do some mining occasionally.

Edit: combine these with a longer duration and proportional output for each /dig (which needs to be interruptible and yoield the appropriate amount of ore on the floor if interrupted), and the system would be very good IMO.