You're buying the salt in the wrong place barry1010. Its currently around £12 for 25kg on ebay with free delivery. That's nowhere near the cost of cooking/table salt with the exception of Saxa which is about £1.00 for a 750g drum.
Mining rock salt is not an expensive process and if anything is on par with salt from underground brine resources in the UK when all factors are taken into consideration. There is far more to it than simple dehydration.