Not huge enough, you can't see the reason of why Southampton first bid didn't include Buyback option to entice City to sell delap to them?.
Because they know they can make huge profit with him and that's why they will try hard to bring Delap to them without including buyback option. They will fail ofc because City stance will clear of this they will not let highly rated academy player like Delap without inserting buyback option.
Unless ofc City is willing to pay around 80m as buyback option then i think Southampton will be more receptive on buyback option for Delap