Mumbo might be able to help you out...
He stocked up our childhood collection to include the whole range a few years ago, he probably has a good idea on its value.
Most of the older stuff is by Action GT, not sure if they are related to Herbert in anyway but here is a Dragon set from eBay with what I would guess is a slighty inflated BIN price.
http://www.ebay.co.uk/itm/Crossbows-And-Catapults-Dragon-Accessory-Boxed-Figures-Projectiles-5622-1983-/361084208711?pt=UK_Toys_Games_Games&hash=item54124bc647
A 'sold listings' search on the bay might give you a good idea.
@Pete WTF is that?