I bought 200mil worth of herblore supplies for this lol. Even though the rate goes down, 1.1x is still better then nothing for herblore.
i think an interesting thing to do for double exp weekend would be, plant your stuff, log off, come back when you know its grown, harvest, replant, log off, and continue, you would only be logged in for a few minutes each time you get exp.
The price increase from people hoarding herbs and secondaries negates any money you'd save by the increased experience earned doing herblore during the weekend.
Gf your cash. If you do get below ~1.5x and are still doing herblore, you're spending more money than you would normally be.