Each pack will cost 800 Microsoft Points ($9.99 USD), but Gearbox will be releasing a Borderlands 2 Season Pass to get all four packs for 2400 Microsoft Points ($29.99 USD) which will offer a 25% discount, effectively making it a "buy 3, get 1 free" sale.
The first expansion is confirmed to include a new class, the Mechromancer.
Borderlands 2 is due to be released on PC, Xbox 360 and PlayStation 3 in the 18th September in North America and the 21st September in Europe and the UK.