I'm not aware of any USB based Scrypt miner that is worth buying.
If you have an Antminer L3+ (504 MH/s) right now -- you are in good shape.
Mine is paying about .02 to .022 BTC every 24 hours. Quite impressive @ about 760 watts.
Only 6000 bucks on Amazon.. Lovely. Don't have that kind of money to invest at the moment unfortunately.