I solo maps as a lvl 70 SMN. I can solo all the maps except gazelleskin.

Gazelleskin goes to the Unzair. Only been there a few times. I recommend at least 6 people.

Dragonskin map disclaimer - I can solo the map - but a dragonskin might open the acropolis - and acropolis level syncs - and I can't solo the acropolis. (I've tried) My FC has done it many times with 4 people - usually no problem. The problem is when Areges randomly drops. Then you might want 5 or 6.

All the other maps I can solo - pop the chest - kill the monters - take all the loot - (why share it)

Some Map prices, at least on my server:
Peisteskin map => level 50 => 50000 gil
Dragonskin map => level 60 => 10000 gil - possible trip to acropolis

The difference doesn't make sense - so it must be as you say, the loot. I'll look into the loot lists. Perhaps I should keep those maps