An arm and a leg
A large, possibly exorbitant, amount of money.
'It cost and arm and a leg' is one of those phrases that rank high in the 'I know where that comes from' stories told at the local pub. In this case the tale is that portrait painters used to charge more for larger paintings and that a head and shoulders painting was the cheapest option, followed in price by one which included arms and finally the top of the range 'legs and all' portrait. As so often with popular etymologies, there's no truth in that story. Painters certainly did charge more for large pictures, but there's no evidence to suggest they did so by limb count. In any case the phrase is much more recent than the painting origin would suggest.
It is in fact an American phrase, coined sometime after WWII. The earliest citation I can find is from The Long Beach Independent, December 1949:
Food Editor Beulah Karney has more than 10 ideas for the homemaker who wants to say "Merry Christmas" and not have it cost her an arm and a leg.
'Arm' and 'leg' are used as examples of items that no one would consider selling other than at an enormous price. It is a grim reality that, around that time, there were many US newspaper reports of servicemen who had lost an arm and a leg in the recent war. It is possible that the phrase originated in reference to the high cost paid by those who suffered such amputations.
A more likely explanation is that the expression derived from two earlier phrases: 'I would give my right arm for...' and '[Even] if it takes a leg', which were both coined in the 19th century. The earliest example that I can find of the former in print is from an 1849 edition of Sharpe's London Journal:
He felt as if he could gladly give his right arm to be cut off if it would make him, at once, old enough to go and earn money instead of Lizzy.
The second phrase is American and an early example of it is given in this heartfelt story from the Iowa newspaper the Burlington Daily Hawk-Eye, July 1875:
A man who owes five years subscription to the Gazette is trying to stop his paper without paying up, and the editor is going to grab that back pay if it takes a leg.
Other cultures have similar phrases; for example:
In France - Ça coûte les yeux de la tête - It costs the eyes from the head.
Bulgaria - Това струва майка си и баща - It costs one's mother and father.
See other phrases that were coined in the USA.