Doing the 34.9 lightweight seatpost search, and it seems there are maybe 3 which aren't heavy pigs, and all are somewhat expensive.
Stock post and hardware is 295 grams.
Many expensive posts are about 240 to 290 grams and about $100-ish.
From my searching it seems 100 grams and $125 is the best.
And a shim such as eXotic Carbon Seat Post Shim 34.9 mm to 31.6 mm Spacer | eBay is about 45 grams before the approximately 180-200 gram 31.6 post at around 100 on average.
I'm the 34.9 newbie, maybe someone can show the way to a reliable lightweight post which won't break the bank. Sure was easier in the 27.2 world.
Mtbr's 2016 Winter Biking GearReviews and Roundups
Results 1 to 5 of 5