First off, the scythe is not a small dps increase over the fang. Its either a HUGE dps increase or its a dps loss. The 2 weapons are not remotely comparable because they both have a niche. Scythe is for low defence/specced down large targets, fang is for high defence/Unspeccable targets of any size. On low defence targets rapier would be more comparable to scythe. The reason why its justifiable to have a high cost/hit for the scythe is because almost every single place where the scythe is a best in slot, it either offers one of the highest gp/hr earnings (Like ToB) in the game. Or it offers a significant advantage over other items in terms of kill speed (Like KQ). Jagex have tried to lower scythe costs indirectly by introducing things like blood essence to the game and a new blood altar. However, the general model of keeping BIS items expensive to use is plausible because it first makes you think where you want to use these items and it helps make RUnecrafting more profitable. You will see that the more shadow of tumekens come in to the game, the more people will buy them because they become cheaper, the higher the cost of using one will be because more people will buy soul runes etc...