back when cloth was high:
5 mino horns turn into an average of 27.2 cloth.
The cloth was worth about 25+g when sold to desperate players.
5 mino horns = 27.2 cloth = 680g
That's 136g per horn.
So basically, every horn they buy from players at 60-70g will give them a profit of 70g themselves.
Now buy 1000 horns, that's 70k profit.
^This was correct like a week ago when cloth was very expensive and sold out at the mat traders.
Today, they might buy the horns at 30g or so, dunno. They'll still make good profits off of it though, otherwise they wouldn't do it.
And you can get to the horn collector a lot easier than the troll tusk collector too =P