You talk about average. there is no average.
To get a +10 reward from your 5 dungeon option you need to do 5 +10's. Any other dungeon's you have done are not relevant. you can do 100 M2+ or 1 M9+, so long as you did 5 10+'s you get a 10+ option.
If you did 4 10+ and 1 9+ you get a 9+ option but the 9+ didn't screw you out of anything, without the 9+ you would have only done 4 dungeons and not gotten the extra item option at all in the first place.
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The people who understand how this system works and still feel they are pressured into getting all 9 options are probably beyond saving and should not be catered to in the first place.