2K Games owner Take-Two recently said that the game was tracking ahead of what the original Borderlands sold at the same time in its lifecycle, but that's less impressive than it sounds. Borderlands was a slow burner that ended up having a very long tail in sales, and it doesn't look likely that Battleborn can repeat that feat.
It's not for lack of trying, though. The game's been out for less than a month, but 2K currently has the game for sale on Steam at a 40% discount. There's two ways to read into that - one is that 2K is hoping for a sales bump heading into the Memorial Day weekend. US gamers have Monday off, so it makes sense to try and push a multiplayer only title to them in connection with an extended weekend. The other way is more pessimistic, concluding that sales of the game are way below expectations and that 2K is trying to catch up by knocking the price down below the $40 price point that Overwatch is selling for on PC.
We'll leave it to you to decide which one is more likely.