Wow, I don't know what's up either but you're right. I checked my past orders at Freestyle and I bought a gallon on May 4, 2011 for $19.99. I bought another gallon later to keep in stock un mixed, just in case since the stuff last so long, just this February for $41.99 and noticed the huge jump then. Looks like it's taken another slight jump.
Given how absurdly long it lasts and how much film it will develop it's still a very cheap developer to use, but I'm spooked by this price increase too because I really like it and like to have it available. Tri-X in Diafine (now at 1000 for tungsten or 1250 for daylight - it seems modern Tri-X doesn't work as well at 1600 in it as the version I shot back in the 80s) is still my combination of choice for any time that speed is adequate, and I've also found, at the other end of the speed spectrum, that I really appreciate Pan F+ in it, at EI 64. The extra 1/3 stop isn't the draw, just where I've found it works best, but it does constrain the highlights nicely in this film that can otherwise be pretty contrasty for use in light that's bright enough for such a slow film.
I'd really hate for Diafine to go away.
I've also been bothered by the fact that a quart would do me much better than a gallon, but the sources that have quarts seem to charge as much for a quart as Freestyle does for a gallon.