Keeping track from my simerian explorer quests.
What they actually sell for might shift but I am just counting with current prices to have some kind of clue of the profit made. Also I am using them myself for few more temple runs.

500 quests
Cost: 3.4 mill
Runes: 23
Runes every: 21.7 quests
Longest without: 62 quests
SS price: 9.75 mill
Profit: 6.35 mill
Average ss cost: 423k per rune
Average cost to get: 148k per rune

500 quests
Cost: 2.6 mill
Runes: 29
Runes every: 17.25 quests
Longest without: 128 quests
SS price: 12.17 mill
Profit: 9.57 mill
Average ss cost: 420k per rune
Average cost to get: 90k per rune

So at the moment the average of how often I get a rune is 19.23 quests. Will keep updating...

500 quests
Cost: 3.36 mill
Runes: 36
Runes every: 13.9 quests
Longest without: 48 quests
SS price: 14.9 mill
Profit: 11.55 mill
Average ss cost: 414k per rune
Average cost to get: 93k per rune