A few things to keep in mind:
(1) Like any case seeking class certification, what happens will over the long term largely hinges on whether the plaintiffs succeed in getting class certification. This is the leverage the plaintiff is looking for to get a larger settlement or proceed to trial under a larger damages model (and I mean significantly larger). If the plaintiff doesn't get a class certified, this will settle within weeks of the denial of class certification. If a class is certified, it will get interesting.
(2) Delta will try to get the case dismissed before the motion for class certification is heard and more importantly before it has to give much discovery.
(3) Don't underestimate the ability of the plaintiff (and counsel) to get outside experts involved (even prior to filing the complaint). This is a pretty common playbook by plaintiff class action lawyers, they pick a case, invest in some technical experts up front and through early stages of discovery and seek to ensure they get past a motion to dismiss and have the facts to support the case should they succeed in getting class certification. I know of cases where experts have performed hundreds of transactions on a web property to in effect reverse engineer what is happening on the back end. And they will likely use discovery to quickly confirm their understanding and delve deeper...
(4) It will be interesting to see if the decisions about willingness to paid are tied to various data about potential purchasers that (as a previous poster highlights) we know Delta has access to. This is the most interesting angle of the case to me, it raises some fascinating issues around big data use in the commercial marketplace.
(5) The "best fare guarantee" may have fine print that effectively tells the user "hey, we don't really mean we're selling you the BEST fare as you would understand that term..." But that doesn't mean that Delta is immune from attack here. There are plenty of successful complaints about deceptive claims where the defendant has tried unsuccessfully to point to clarifying or disclaimer language.