But can £20 be considered a fair price?
I would. I think the ones I bought back in the day cost around the same money brand new. So I would say it's fair based on the fact that they haven't taken into account inflation and that its quite a niche product these days. There aint that many beige drives around or people buying them. You wouldn't be buying one of these unless you wanted it for a retro project so its basically a collectors item.
You can still buy slimline external optical drives on Amazon or ebay for the same money or less