How does Bitcoin Gigahashes Convert to Ethereum Megahashes? 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.
Bitcoin and Ethereum use different hashing algorithms. General consumer hardware (like a CPU or GPU) can be easily programmed at a software level to perform either the Bitcoin or the Ethereum (amongst many other) hashing algorithms. However, specialized hardware (like the Block Erupters you mentioned, which are Application Specific Integrated Circuits (ASICs)) have the hashing algorithm hard-coded into the chip's design, allowing it to much more efficiently perform one type of work (Bitcoin's double-SHA256, for example) at the expense of flexibility--they can't be reprogrammed to do anything else.

On a side-note, a £60 ASIC which runs at 16 GH/s (which isn't a block erupter, it might be the USB Stick from GekkoScience?) is very unlikely to ever reach ROI. At current difficulty and Bitcoin price, and assuming free electricity, a 16 GH/s miner would bring in about £0.26/month, which is likely to get even worse as more mining power comes online in the future.