just off the top of my head...
Are the prices you mention for AT power supplies? Because seeing as how they are old technology you could very well pay extra simply because they are harder to find and not worth keeping in stock for all but speciality retailers. Also, during the time of AT power supplies, the 400w range was usually reserved for pricy servers so finding one in that range will be costly while for ATX, 400w supplies are the norm these days if not considered small so finding an ATX 400w would be cheap.
You are correct that ATX supplies usually need to be connected to a motherboard but if you are technical enough to hook some bare drives up to a power supply on a shelf then you may be able to handle getting an ATX supply to power up. Basically, the ATX supply has some pins on the mobo connector that can in theory be shorted out to get the supply to power up; Now I couldn't tell you which ones because I only saw a reference to it once in passing but I bet you could google for pinouts of the ATX's mobo connector and figure it out.
Maybe that helps, sorry I can't really give you more specifics but you did ask for thoughts and suggestions so I shared mine