Jim, I'm not clued up on what current prices are for writing boxes. You do get a lot sold on EBay, but I wouldn't buy that way, there's too much you really need to see up close to check on condition. I did a lot of looking around before I bought either of my writing boxes and found that the quality of boxes varied considerably .
I've no idea of the date of my walnut and brass banded one. I bought it for using and so was just looking for something very cheap - it was £100 about 6 years ago. It has leatherette on the slope (horrid) and had no key for its lock (my dad cut a key to fit the lock). It's larger than the box I've photographed here and heavier, the latter being the reason I wanted to get a smaller box. I do tend to write with the box sat on my knee.
The ebony box was £170 and I bought it at the antiques fair at RAF Swinderby (hope I've given its correct name, it does get nicknamed Swindleby because not all of the stallholders are as legitimate as the writing box man I bought from

). I liked the look of that box because it's ornate and yet also practical. It's got very heavy duty hinges and also a heavy duty lock. Also, it had two "catches" to hold the slope in position when the box is closed, so many boxes just have one catch or little brass clip - I always think the latter look cheap. I thought the two catches were good because you'll see so many boxes where the wood is warped. Look at the bottom pics on this page -
http://www.hygra.com/wb/wbnwb05.htm - you'll see the slope is actually made up of three pieces of wood. That's how the box should be constructed, in a lot of cases the slope has been replaced with a single piece of wood and that's not as strong.
There used to be a website for an antique shop in Warwickshire which gave details of what to look for in a writing box, as well as things to avoid, but I can't find the link.
Might be useful for an idea of some prices -
http://www.portobelloroad.co.uk/specialist/boxes.aspWe used to go to Newark quite a bit for work and there's other antiques fairs from the same organisers. No idea what the others are like, but Newark is the biggest in the country, you go first time and walk round in a daze trying to take it all in, next time you go you learn how to focus. We were lucky, on our first visits we had a couple of antiques experts with us, but even so it's quite a daunting place just for its sheer scale! Haggling is a must.
http://www.dmgantiquefairs.com/