Where to buy supplements on discounts? (Borderlands)

Without going into details:
there is a game I want to re-take. Want to re-take with all the DLS, which I have no. So historically that 2K is not doing discounts on additions to the first Board. Itself, then you can buy it for $ 100 for every sale. But separately buy all 4 DLS will be released in good 1300 Rublev. The incentive is GOTY Year pack (game + 4 dlc) but it can not be purchased if a copy of the game is already in the library and it is there. Synpase for the game too, no...
So, if we buy additions to the game separately and at a discount?