Wk 1 tickets have held relatively steady in the $500-550 range (not incl fees) on Stubhub since the lineup was announced. The last half of February they've dipped below $500 a few times, but have usually bounced back up over that mark pretty quickly. It was usually only a few listings that got below the $500 mark before they sold.
The last few days they've started to drop more quickly into $475 territory, and this morning the low got to $450. There are now a few dozen tickets under $500, and the market is definitely shifting significantly downward. That should accelerate in the next 2 weeks when people start actually receiving their tickets and putting them up for sale.
Last year followed a similar pattern, and I would expect the Wk 1 prices to continue to drop into the break-even range for sellers ($425-450 depending on what they bought at) by mid-March. Wk 2 have already hit that mark, and are likely headed sub-$400 pretty soon. As always, don't even consider buying until late March. Craigslist listings are increasing daily, and it'll be flooded in a couple weeks. GNR and LCD are not going to drive demand, and getting Wk 1 at face and Wk 2 under face should not be difficult within a week of the festival. The reality now is that a LOT of people are buying pre-sale and then trying to unload in March and early April, so outside of 2014 it's consistently been a buyer's market close to the festival in the 2-weekend era.
Tip on using Stubhub as well: the lowest priced tickets or shuttle passes are not always listed at the top of the scroll, even if you sort for price. People sometimes mislabel the section/row as something other than GA, and those get stuck down at the bottom of the list, even if they're the cheapest option. Right now, sorting for price for shuttle passes Wk 1 gives you $43 as the cheapest. Scroll down to the bottom and you can find them listed for $33.50. Same trick works with actual tickets.