I've done some stat on the SGQ 80 and Manyu Spirit Quest 80:

Manyu Spirit Quest 80 stamina:
28 runs
Lv100 : 14 => 0.5/run
Lv20 : 66 => 2.4/run
Lv5 : 31 => 1.1/run

Secret Garden Quest 80 stamina:
31 runs
Lv100 : 44 => 1.4/run
Lv20 : 66 => 2.1/run
Lv5 : 0 => 0/run

This was before the "bonus" in the manyu quest.

As a side note, I have ~55% of chance to get the center chest of SGQ.