When you enter the barter screen after getting the discount, Renesco's stuff (except the money) may be worth $0 right away. It can also happen that the cost is merely reduced by a factor. In my case that factor was initially 5; however, after I left the barter screen and re-entered by telling him again I wanted to make "purchases", the prices were reduced by a factor of 100 instead (this is because the discount modifier is applied each time you enter that dialogue node). By trading things back and forth, you can quickly and easily get the whole of Renesco's inventory for a mere $1 (including his cash - trade something for it which you can then get back for free).