Case mods are always an odd one to try and price up. I suppose the main question is how fast do you want to try and shift stock and secondly are they going to be premade designs or something custom?
High prices will sell for the same reason that white headphones still sell better at higher prices and more so if there is a shortage of drives (banning waves and people shifting the cost of an arcade are good for this).
Low prices will shift fast but obviously with lower profit.
The two obvious things to look at are 250 gig drives prices (set to be released as a standalone item soon and for once at a price approaching sensible although still a nice margin if you are going to do mod of a standard drive) and the spare cases (there is a fair difference between DX and more local shops).
Also is it a pure case mode or are you going to add a basic sata port/extension and power options to hook it up to a PC because that would mean a fair bit to people.
In the end price of a standalone 250 gig drive + 20 to 25 is what I would look for. More for custom and sata addons.
