Like anything, it varies based on supply and demand. When a lens is discontinued because a direct replacement is released, and that replacement is a substantially higher price, That can drive up demand for the old version, meaning prices rise accordingly. That's what happened with the 70–200 II and the 24–70/2.8 II.
The 24-105 has been a kit lens for many years. That means there are LOTS of them out there! The typical used price used to be $800, which was what the lens added to the kit price. Recently, that kit price add dropped to $500 for a a while, and now it's commonly $600. Even if the lens is discontinued, and the price goes up, I highly doubt it will go up by 33% to bring it back to $800.
The price on the 24-70/2.8 II will probably continue to drop over time, and that will mean even more 24-105s on the market. I'd say if you really want to sell it, do it soon and cut your losses before they get worse. Personally, I'm quite happy that I sold mine a few months ago, when the going rate was still $800.