Where I live its extremely rare to find an Imac .
In the US, a 15" iMac G4 in that condition would be a tough sell at much above "come and get it" pricing.
These were a great and very innovative model from Apple with tilting/swiveling LCD. Most of the market interest now is in the later, faster models with 17" or preferably 20" screens.
If you get it clean and properly running with an replacement-DVD-drive, maxed out RAM and Tiger&Classic/os9-combo it's a fine machine and you might score here 100-150€. But it all depends on the local market ...
But the most valuable thing about it is to use it yourself and have fun!
10$ for the machine and most software is practically for free!