I have a mod that freezes time at midnight, so i can spend as much time as i want in the Mines/Skull Cavern. I entered the Mines while they were Dangerous, and spent the next 2-3 hours (RL) clearing every floor of enemies while under Monster Musk. By the time i reached Floor 120, i had 2 Galaxy Souls. This was with a Bandit's Ring equipped for double drop chance. The second time i did a Dangerous Mines, i did the same, and received two more Galaxy Souls. Was able to upgrade my Galaxy Sword to Infinity Blade without spending any Qi Gems on Galaxy Souls.
The drop chance is really low, considering how many monsters i slaughtered with a Bandit's Ring on. The mobs can also drop Qi Gems as well, at a somewhat better rate.