If you're looking for an affordable and cheap computer gaming mouse that still delivers, you can do far worse than a Logitech MX 518 gaming mouse. It is the successor to the Logitech Mx 510, and has been endlessly polished by Logitech over time.
It has a polished holographic image look on the mouse, that on top of its shiny colors, appears to have indents. It can be described as bullet marks on a metal sheet. There is a 1600 dpi version and a 1800 dpi version, but both are still good.
It is known for being an ergonomic and easy grip, supports on the fly dpi change, and many many programmable buttons. Being out there in the market for so long, and still being in there, pretty much all of its kinks have been worked out.
Its main selling point is its price range. Do note that it is optical instead of laser however, and thus cannot be used on certain services such as certain shades of red. Its also rather large, and a claw grip might need some getting used to.
It is the cheapest modern gaming mouse out there and is the best value per buck spent. A proven design, affordability enhanced due to economies of scale, and built as durable as a tank, it can last many years.
Even if you happen to be using a pricey main mouse, an mx 518 always serves to be a good backup mouse in case anything happens.
It is used by quite a few modern professional gamers these days. At least until they obtain a sponsorship with an endorsement company that would produce pc gaming mouse.
Take care not eat potato chips and smear your mouse, as apparently on the forums that is what a lot of people do, and it ends up eating into the rubber skin of some of the grip regions.
It works pretty well for most FPS, and does decently in RTS as well as MMOs.