(50^10 + smaller terms) * 256 bits (SHA256 size) Number of hashes * size of hash to store.
Obviously this isn't how it's actually done. The best page I found describing the process is: http://www.freerainbowtables.com/en/faq/
The forum is fascinating: http://www.freerainbowtables.com/phpBB3/uncracked-hashes-f23...