They probably aren't that expensive to manufacture but they are £163 inc touchscreen. The rest is customs/postage etc.
Unfortunately its the size vs the res it can display which puts the price up.
I don't want to sound as if I'm singling you out but your 17in TFT monitor is probably capable of what 1280x1024 max. These 7in VGA widescreens are capable of 800x480 and are less than 1/6 of the screen area (6.1x3.43in vs 13.6x10.2in for your 17in screen) . i.e they use TFT panels that have a smaller dot pitch than a conventional PC TFT screen which IS harder to manufacture. You try finding out how much a 17inch TFT screen that has a panel res of about 1800x1400 costs.
I do agree that what appears to be (woudn't know mine doesn't seem to have any image and the amount of ****ing about I've done with powerstrip its been out of sync a fair few times

) the 2nd hand nature of the panels suck big time, but as I said its probably the cost of the panels that meant they have sought a cheaper source. This is also why I feel the Xenarcs are more expensive as they are no doubt brand new panels.