Yep, definitely between £500 and £1200!
Have a look at similar bikes in Motorcycle News, Auto Trader and on ebay and make a judgement about current market value accordingly.
Prices will be on the up, as summer is on its way. No really, it is.
IMHO, it comes down to three things - apologies if you already know how to suck eggs...
1. How desperate you are to sell, or not. If you need to shift it quickly, advertise it at a knock down price. If it doesn't matter, aim at the high end of the price range and see what happens. You can drop the price incrementally until you start getting some serious calls.
2. Make your bike look like new. Spend the time cleaning and polishing. If there's anything wrong with the bike that can be fixed, fix it. How many times have you seen a car or bike advertised with words like "needs a new mirror, easy repair" and thought, if he can't be arsed to repair it before advertising it for sale, what else couldn't he be arsed to do? Then take photos from every angle. Get some close ups of the good bits! Take way more than you need, then pic out the best ones for the ad. If you took some with the camera on its end, turn the pics the right way up before you post them!
3. Your ad. A carefully thought out and well worded ad can make all the difference. Good pics from every angle are essential. Make sure you stick to information relevant to potential buyers. Don't waffle on about how brilliant it is and how you fell in love with it when you first rode it, or whatever. Buyers want to know how much MOT and tax there is, the current mileage, the condition and how much service history there is. Does it ride like new? Is it mechanically faultless? What's it like cosmetically? Have you replaced any expensive items recently, or had any major services done? The CB400 is an excellent first bike, so a few well chosen words about how cheap it is to run and insure might help, but most prospective buyers will have done their homework and know this already.
Seriously, if your bike stands out from the rest of the bikes for sale, you'll set it for top money.
HTH. Best of luck, but I reckon if you take my advice you won't need any! :D