What websites are you currently looking at cause from what I see thats not how it goes. 256mb versions are always more expensive than their 128mb counterparts. All the specs on the graphics cards are exactly the same except obviously the memory. I dont see how 128mb to be more expensive is possible.
I have not seen what you are talking about on any website, so far anyway. If you are comparing 128mb cards to 256mb ones pricewise you'd have to choose 2 by the same maker eg 2 MSI cards and - this is the important bit, you must compare like for like. There is no point in comparing a FX5600 with a FX5600XT or with a FX5600 Ultra. If you can give me an example of 2 cards which are both the same except for the amount of memory and which the cheaper one is the one with more memory I would be very interested.
Couldnt of explained it better there Totally-braindead.
All I can see is the reason why some 128mb cards are more expensive than 256mb cards is due to the fact they are more recent and more powerful models a.k.a FX5600XT 128mb compared to a older FX5200 256mb.