Hello there pete.
I have one, its in pieces though but I’m getting there.
I’ve tried to reply twice but its eaten my message both times so I apologize for being brief.
Prices are tricky because it depends on who wants the bike. Going by ebay I’ve seen similar bikes go for completely different money. £300 sounds quite cheap even if the bike is a wreck. Good condition prices even trickier because in the last year of watching out interest on ebay I’ve only seen one bike that wasn’t a project!
There are two different models they look like the 400 older with two headlights and a newer one with a single headlight. Chassi wise they are relatively similar.
Parts to answer Dave6790 question are hard to find but turn up occasionally when ebay breakers split bikes. They can turn up cheap because they are taking up space and so few people want them. I got a set of mint forks for 30 quid once. Parts aren’t similar to the 400 hundred at all as some people assume which can also cause trouble. I’ve found that you can fit a modern r6 shock which seems to work quite well.
Performance I cant really answer as I haven’t got to ride mine because it was rough when I got it. Older style bikes make around 40hp I think and post 91 models make roughly 45 I’ve seen a dyno chart from a 91 bike that made 43hp after 94 it was restricted after change in Japanese law, not sure how presumably with the cdi like the 400. Its also speed limited in japan so you may need a box that does that, quite cheap as it’s the same for the 400 derestrictor box.
Try these links:
http://www.geocities.jp/zxr250_owners/" onclick="window.open(this.href);return false;
Search for zxr maniax on google so you can translate it.
And
http://www.zxr250.com" onclick="window.open(this.href);return false;
which is a uk site that has a forum. People on there may be able to help you with prices.
Cheers martin.