I don't usually post individual deals, but this is a fairly big drop in price for a very popular game. The physical copy of Fallout 4, for PC of course, is currently a little over $18. Since, digitally, it is still a $60 game, this is about 70% less than the price on Steam.
I'm guessing that this deal is to clear out stock for an upcoming Game of the Year edition. This is something to keep in mind. The last DLC has just been released two weeks ago, and, if history serves, that means it won't be too long before they release the game with the DLC bundled in. Probably, if you waited this long, you should wait until that gets released (and goes on sale) to pick it up. I doubt that it would arrive before 2017, though, so it's up to you.