I have to laugh when I see the great data recovery ‘challenge’. Lets be honest here folks, businesses are in it to make some level of profit from their efforts. To that extent they have facilities in place to recover data from damaged drives due to a variety of problems from simple surface level damage all the way through to failed drive electronics (swapping out logic boards).
The price quoted is generally based on the amount of effort that needs to be gone thorough. Accidental erasure is probably the cheapest. Simple disk-level damage (e.g. a few dodgy sectors) can be resolved using tools like Steve Gibson’s Spinrite; which is pretty much a good example of what these companies would be doing. Drive electronics failures would cost more – for example they may need to disassemble the drive in a protected atmosphere to replace something. Large scale physical damage to the drive may entail extracting it from the original housing and essentially replicating the internals of the drive in order to read the data from it. This would be very expensive, but would succeed in the face of quite significant damage.
The intentional erasure of the data using utilities like dd are pretty much a non-starter. For the first part, you need insanely expensive specialist equipment, the rate of data recovery is slow (we’re probably talking in the order of bits per second) and the chances of actually recovering anything useful on a typical hard drive is nil.
For any typical person trying to wipe their data any of the secure erasure utilities available for purchase or for free are more than adequate to prevent the data being recovered by any agencies.