Depends on what is wrong with it.
21 pounds
As of this writing, the 3.5 GHz 27-inch iMac (with a 5K display) retails for $2,499 USD.
Because the iMac renders colors much better than Windows PC. The iMac is also much easier to use for graphics than Windows PC. It is also more reliable and has far less crashes than Windows.
Apple no longer produces a 20 inch iMac. You would have to buy a used one to get a 20 inch model.
No, you can only get that discount on a new iMac not a refurbished iMac.
You can get a refurbished iMac with full warranty on the Apple website for as little as $999. This is as cheap as it gets for a new iMac. Check the refurbished section and if there is none there check back each day.
There is no "iMac Air".
The original iMac was the G3 which came out in (1998). It was followed by the G4 (2002), then the G5 (2004), then the Intel iMac plastic version (2006). The 5th iMac was the aluminum iMac which came out in (August 2007), followed by the iMac Aluminum Unibody (2009). The newest iMac is simply called 2012 iMac and it came out on November 30, 2012.
An iMac is a computer, but a computer is not necessarily an iMac, so they are not the same thing.
The iMac
The original iMac was created in 1998.