May I ask how much gold do i need to buy Twig??
It depends on your choices through the encounter! If you choose all the right options Twig's master may sell her to you for just 200 gold, but if you act poorly it might go as high as 600 >:)
Just to give you a couple of hints: you should introduce yourself as a noble (as he would want to have a good relationship with a possibly rich client), and you should definitely make sure he doesn't think you've talked to his property before (it is very disrespectful to approach other's slaves...)