It doubles each time. 10 20 40 80 160 320 640 1280 2560 5120 10240 20480 40960 81920 163840 Etc. You're 20th mw core would cost a little over half a billion shards. Or if you're patient and buy 1 each day would cost 200 shards. Edit: around 5 million+ at 20. 5 billion+ at 30 cores.