It's an interesting range and I will be sorry if it is no longer available. There are a lot of 'would be nice to have but not essential' models in the range. It's a good website now too.
The topic of valuation is an interesting one. As pointed out the only real guide is actual profit and if sales have been few, then clearly the business isn't worth too much. It depends what comes with the sale: as well as the photo etches there are likely to be moulds for whitemetal and brass castings. All of these have tangible value. The price of the intellectual property in the designs is more difficult, but if you look at the labour costs in drawing up a design, having test etches built and then redrawn where necessary, there is certainly value there. Goodwill is much more difficult to price. Stock can only be valued at cost price if unsold, and as others have pointed out, if nobody is interested in buying, then the value is only that of the recoverable items and then scrap, so not much at all.