Are you buying or selling ? It's worth what you can get for it or what you are willing to pay for it is the only answer you can really expect. I know what mine cost new and I would not sell it for what someone would probably be willing to pay me for it. I would say if you are selling set it about half what you paid for it and see if anyone bites, £400 and near offers something like that. I know mine is a great little tool but now the 200 Evo is out it must be taken into consideration that they are outdated even though they are still up there with the best in my opinion.
When you say good condition what do you mean? has it got scratches/dings on the case or not, do all the knobs work smoothly, are all the leads etc in nice nick?
Also consider how old it is.
If all the aesthetic's are okay & its in full working condition + you've got all the leads you need like earth, electrode holder etc..
then you'll probably get around £275-300 for it if its just the stick machine with the TIG amphenol plug.
If its the MinarcMIG you're looking at around £400-£450