Mirrors cost way too much, cost of transport also seems to be overstated...
The base cost might be due to the skill required to craft one with extra charge from monopoly they hold.
Oh and it being a luxury item since there's already bronze mirror as cheaper alternative.
The cost of transport make sense if you remember that it take multiple teleports to go between these two town and without his MP absorb sword the journey would take much longer meaning more cost to stay/food.
Not to mention the fragility of mirrors mean one fuck up and you're down a huge amount of money.