About how much to replace iMac Intel 20-inch motherboard?

Question

Hello. Can anyone (in the UK) tell me about how much it would cost to replace the motherboard on a 20-inch Intel iMac? Mine is slowly dying (details below), in particular with graphics problems. Thanks.
More or less from the outset, my 20-inch iMac had an intermittent fault where occasionally it would start or wake up with major graphics problems (see http://tinyurl.com/iMacFault1). As I'd bought it from an Apple retail store, the only thing they would accept is for me to bring it back to the store (difficult, in my case) and have them examine it. I've been there before: I'd go to the trouble of bringing it back, they'd test it, it wouldn't fail, they'd say there was nothing wrong etc. They weren't interested in the photographs of the fault. So, I let it go.
Of late, it's more often suffered from a slight shimmering with video (which you can often clear by powering down and up) and/or rows of corrupted pixels in windows (see http://tinyurl.com/iMacFault2). Very recently, it's started to freeze completely.

Answer 1

The cost is prohibitive to the point you would be better off getting a new machine and selling the old one on eBay for parts. I don't recall the exact amount, but a new logic board for my 17" iMac was somewhere in the $650 range (~409 £). Apple should be able to quote you a price for the replacement to give you an exact idea.
If it is still under warranty and freezing consistently, go back to the store. iMacs aren't cheap to replace so you may try insisting they take a harder look at it. Best of luck!