Someone's desperate if they want to pay more than 500k for a 105; hell I had other fishers offer me one for free, but I chose to earn my own (on a very low pop shard, at that) - besides, people who ALREADY bound the scrolls to get a 110, are reporting that they are getting 110 scrolls dropping from quests (a probable side-effect of how they changed the quests for dungeon fish to 106 skill, without double-checking the value needed to get a 110 scroll).
So, a 120 fishing scroll would not be 10 billion....
More along the lines of 100 million, for someone so desperate buy to combine all the scrolls necessary, including fishing their own.
They'd be better served training up to 110, try to get the scrolls to combine to a 115, and hope for a 120 scroll drop after the changes.