I bought 6 XFX Rx 480 8GB graphics cards and I estimate they will do 25-30 megahashes with Ethereum.

When looking on Amazon for other rig ideas I was reminded that they also sell Erupters. The erupters I was looking at are £60 roughly for 16 gigahashes. Not sure if that is only with Bitcoin or not but most of them say Bitcoin in the name/title of the product in sale.

I was wondering what this would equate to in ethereum megahashes and why?

My guess would be that it is something to do with the the difference in complexity with the encryption method, or something like that, but of course, for economy reasons the most important thing I would like to know, if possible, is what hashrate they would generate for Ethereum.

Thanks for any help in advance.