Might be that they're unusual or new varieties.

and people will pay for them too. but even a small rhizome that will flower will have had to be nurtured for a few years. and that all adds to the cost.

I grow colocasias - these are known as taro in asia and are eaten (similarly used like our potato here). So you can buy Colocasia tubers or a growing plant from a nursery, for say £6. Then if you can find Taro (or eddoes as the tubers are called) in food shops - they don't cost half as much. I found Colocasia esculenta tubers as eddoes in my local ASDA. I purchased two of them for just 75p each - and of those two one rotted, but one grew into a lovely plant - so if that's what these nurseries do then where's the nurturing there?

The rhizomes I saw were just fairly common varieties, loose in a display box alongside the tulips and other items, Wouldn't have minded the price so much if the size had been acceptable! Have since bought at a slightly better price on ebay. Wonder what spring will bring forth? Meanwhile have divided what I already have to spread them out a bit more. Bearded iris are my latest craze, though I have always liked them.
Yes Alocoloman, I've grown loads of things off the fruit and veg shelves at the supermarket. Satisfying, isn't it!