You'd do better separating the clutch from the gearbox and selling it separately IMO...
Regarding value I'm reluctant to say what it's worth as any potential buyer here wouldn't thank me if my estimate was too high for them.....The seller is supposed to fix the price after all...
However, I would say that £400 for a gearbox is considerably more than I would pay personally at the moment...Unrestored swinging arm BSA boxes, which are equally sought after, can be bought at a jumble for £150-£200...
Gearboxes generally have gone up in price but as usual the asking price tends to follow the last highest price seen on e bay and I feel many boxes are over priced as a result...
Good examples of M20 boxes are getting harder to find and that affects price but I find the market today is so variable that 'This thing costs this much' doesn't apply in the way it did in the past...E bay bidding, for example, can result in huge price variations from day to day...Ian