Using Saradomin Brews as an example, Saradomin brew(1) is 3,307gp Saradomin brew(2) is 6,427gp (+3,120gp for second dose) Saradomin brew(3) is 9,794gp (+3,367gp for third dose) Saradomin brew(4) is 13,200gp (+3,406gp for fourth dose) Since there are a lot of other factors in those differences I would just approximate +3,500gp for five doses and +3,600gp for six doses. Therefore I am guessing: Saradomin brew(5) will be 16,700gp Saradomin brew(6) will be 20,300gp With some more demand in general because of their advantage in terms of trip length. EDIT: Oh I misread, you mean just the empty flasks themselves. Disregard ^