In terms of gaming mice, $40 seems to be the sweet spot where you can get excellent performance for a relatively low cost. We're looking at some $40 options that will satisfy the needs of most gamers.
First up is the Logitech G203 Lightsync. You knew it was coming. With incredibly RGB lighting and a classic shape, this mouse is a great budget option. It has all the buttons most people need, and a nice scroll wheel to boot. The lightsync RGB can match to your screen, the surrounding sound, or be programmed in any way you like.
Next up is the Razer Viper Mini. This is mouse is one of the best ultralight mice out there today, and it's just $40. It comes with snappy RGB underglow and optical switches. Like the G203, it has a 6-button layout, although with a very different shape. It also weighs just 61 grams, an amazingly light mouse that rivals other, more expensive options such as the MM711 or any option from FinalMouse. The optical switches are extremely durable, and although the shape, in my opinion, is not great, the smaller size will give superior control to younger or smaller-handed gamers.
When it comes to inexpensive, durable mice, Steelseries is surely one of the best brands. With their Rival 310, they have combined performance, durability, and comfort into a $36 package. I like the silicone side grips, which give you some extra control and are nice and soft. The mouse switches are rated to 50 million clicks, and the TrueMove 3 sensor is an excellent sensor more commonly found in $65+ mice. I also appreciate that this mouse is 88 grams, an easily maneuverable weight that's easy to get used to. The Sensei 310 is $43 if ambidextrous is more your thing.