Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
What would a imac 24 go for???

Get the Model Identifier and some specs (GHz, RAM, HDD, GPU) from your neighbour's iMac and look up more specs here, then go to Mac2Sell or eBay and look for that model via specs (example: 24" iMac, 2.8GHz, 2GB RAM 500GB HDD) to get a bearing, as there are many 24" iMac models to get a bearing.

Example result 1: 670 GBP
Example result 2: 850 GBP
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.