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The theory is the more expensive (Supposedly better) cables are finished better. The cable itself is pretty much the same. How the cable is terminated is what you pay for.
All this said I have been unable to detect a difference between a £5 cable and cables costing 20x that.
Cheap cables are have, well, cheap connectors and use cheap plastic as light path, thus increasing the risk of causing reflections or signal degradation to some extend. Not sure if it makes an audible difference though ...
I would just get a cheap one from ebay £2 or so for starters (search for toslink cable but watch out there are two types of connector). After all its only light being shone down a a glass pipe, if it was so critical then how would all the cable companies get by with the distances they transmit.
Save the money and go out for the night with the saving.
Mo, I've not been onsite here for a few weeks and missed your question :/
I did a vast amount of research around this topic about a month back when I got a new amp with fibre inputs. The conclusion I drew was based on a lot of good content from a few sites. I've started a couple of pages on my personal website below, specifically based around SPDIF (digitial audio interconnect), conversions and their interaction with cables.
I basically drew the conculsion that 'cheap' cables (most of them) use the same cable (2mm PPM) with a range of ends. The higher the price, the better the ends and their finishes typically. I haven't bothered chasing down true glass fibred cables as SPDIF doesn't require the bandwidth that it offers. Decent cheap cables will do that happily.
At this stage the page is a little sparce but it does at least have a good selection of links you can jump to fully explain cable types available and the finer details of the conversion from TTL to both light (Fibre optic) and/or current (75Ohm Coax).
Head on over to the /doodads site and look under Home Stereo menu.