Simerian Explorer:
A sample of 1,000 quests to see an average drop rate for Runes. I hope this may prove useful somewhere ...

#1 - 100 Quests - 8 Runes:
B / C / F / H / J / P / T / Z
Drop Rate: 12.5

#2 - 100 Quests - 6 Runes:
K / L / M / P / R / U
Drop Rate: 16.6

#3 - 100 Quests - 3 Runes:
N / N / O
Drop Rate: 33.3