Apple dont want to mend your old Mac.
They want to sell you another one.
So the price is ridiculous.
Sony & IBM owners take note, this attitude to pricing is not exclusive to Apple.
So back to the iMac:-
Logicboards for iMacs vary greatly, but few top £400.
The labour cost should not be any more than £100
If it does, ask the "engineer" why it takes SO long to replace a board and listen to the excuses.....
Any repair that is more than 50% of the current value of a working example, is highly questionable.
If you have a black-back or silver back iMac , you have a machine with some value, even if its faulty.
So consider flogging the broken machine for spares.
If you have a white iMac, give it away. Its not worth the hassle of flogging it.
With Mr Visa and Mr Barclaycard about to remind everyone that Christmas comes at a price, there are about to be some good deals on 2nd hand kit......