The Borderlands 2 Mechromancer DLC is to be released on October 16 for 800 Microsoft Points (around 6.99 GBP).
Anyone who pre-orders Borderlands 2 will get the Mechromancer DLC as a bonus, which is particularly handy for those who aren't shooter experts, since it's designed for those who are new, or just not confident with the genre.
As previously reported, Borderlands 2 will also get a Season Pass which will include the four DLC expansions which are set to release over the course of this and next year. The pass does not include access to the Mechromancer DLC, so you'll either have to pre-order or pay to get it.
You can pre-order Borderlands 2 below.