In my honest opinion, stay away from razer brand. It's mainly high priced mouses with terrible firmwares and softwares. Most of their blue prints were released and questioned many times with no answers from the CEO of razer. Even most of their "GAMING" keyboards are terribly designed and I even recall one of their models wouldn't allow more than 2 keys to be pressed at the same time.... which is quite ridiculous for a gaming keyboard that sells for 80+ bucks.
One of the most accurate mouses to this day is no longer made and it's a shame But the G400 looks like quite the replica of the MX500, I personally have only used it at a friends house and it seemed like a nice responsive mouse and i believe they are rather cheap too. I'd definitely recommend it, especially if it's got all the features of mx518/mx500 and more.
On a side note I haven't tried any of the other side companies like ThermalTake, Gigabyte, Steelseries, and Saitek(cyborg). But I have heard they are quite decent models from each company, I wish I could try them out at a store but I rarely see gaming mouses anywhere now adays.