I have done some serious thinking about this, due to a sizeable army and several investments.
Right now, herbs are crashing. However, check a 180 day perspective, and almost all of them are actually higher in average.
Before the double xp weekend stuff is going to go up. However, everyone's hoarding the herbs due to this crash, so within a few hours of the price going up the market is going to be flooded again.
A week after the xp weekend, things should be better. I don't think the price will climb, but it will stay stable, if a bit lower than before.
As more and more people start SG bots, it is common sense to assume the herb price will slowly fall. Say 5% every 2 weeks. However, as this happens, training herblore will become more and more of an attractive offer to other players, so more and more people will be buying the herbs. I reckon this this will be sufficient to counteract the slight drop in price.
tl;dr - After the xp weekend they will crash, then hold steady. Maybe *slowly* decline.