For example - you can buy 8GB of storage in a microSD (SDHC) for a fair bit less than £30 including postage. That comes with a usb key style dongle which must be worth a quid or two.

With an appropriate SATA controller, you could combine 4 of these with their 512 byte block size to give you a 2KB native block size SSD of 32GB. Since all you're doing tech-wise is building a single block from 4 native blocks, you can most likely just multiply the native speed by the amount of cards in use. Write speeds would be in the region of 60MB/s, read speeds in the region of 100MB/s.

All that's missing is the SATA controller with 4 microSD slots.

So why does a 32GB SSD cost around £500?

I was under the impression that the type of flash you get in memory cards and USB dongles just isn't durable enough to act reliably as a primary storage device. But maybe I'm living in the past?

Same AC :-)

As far as I know, it's mostly the same stuff. I have devices which are reflashed completely many times a day and work for the 6 months or more we're developing ROMs without any problems.

If there is a difference, then it will be in the amount of spare blocks provided for faulty block replacement. That's not an insurmountable problem either.

Even if you were to quadruple the amount of flash cards you use you would still not reach the final price of an SSD. And that's at retail prices for flash cards. Remember that a lot of the cost of a flash card is in the packaging and they also include their own controller for bad block remapping, ECC etc. You could do that in one silicon block if you have a direct flash connection rather than an intermediate interface like SD.

I'm pretty sure (although I can't find a reference right now - typical!) that I've seen average lifetimes quoted in years for these cards in 'normal usage' whatever that is.

I think SDHC cards are supposed to have a minimum 100,000 physical erase cycles per block - extended up to 2 million plus through the use of ECC and remapping, known as logical erase cycles. I've even seen some SDHC cards which claim a physical erase cycle count of 300,000. That's a lot of cycles.

It's got to be an attempt to milk the price as long as possible until scale kicks in. Hopefully this is the first step towards those scale economies.