I know I'm like 4 years too late for this reply to be relevant to the OP, but I've had a problem with the USB cord as well and I solved it in two different ways. This is basically for anyone who tries to find any information on it online (there is very little to no info on it).
1. This option:
- is time-consuming
- will teach you a little bit about soldering
- is cheap
Cut the damaged part of the cable out and solder it back together. Soldering is quite easy (succeeded on my first try) though the cables inside the USB cable (cablecaption, I know) are very thin, so you need to be extra careful. As this part of the cable will inevitably be weaker, I suggest buying cable jackets/tubes (don't know exactly how it's called in English, the cable jackets in question shrink when the heat is applied, so that they wrap the cables tightly after soldering) and insulating the soldered part. Fun fact: buying the soldering kit (together with cable jackets) is still much cheaper than buying a new G400 (go figure).
2. This option:
- is even cheaper
- is super fast and easy
Buy the cheapest logitech mouse out there (I bought logitech B100, which should be about 7-8 $ or cheaper). Replace your damaged cable with the brand new one from your cheap logitech mouse. That's it (yes, it fits).