If you're building a budget system, the best favor you can do yourself is buy an inexpensive PC case that explicitly does not come with a side window. Whilst a case with a window might not cost all that much more, we can bet you that your shopping list will end up much more expensive once you give in to the temptation to fill it with RGB-enabled parts. One example of such a case is Thermaltake's Versa H15, which we've spotted to be available for just $25 on Newegg after a $10 rebate card. It even comes with a 1,000-RPM fan and free shipping!
Thermaltake Versa H15 Micro-ATX Case: was $45, now $25 w/rebate
At this price, your wallet will thank you for not splurging on an expensive fancy case, especially because you'll also save money not fussing about the looks of the parts inside! With the $10 rebate, it's currently selling at its lowest price yet.
The case comes with a pretty standard internal layout, featuring a Micro-ATX design (note that you won't be able to fit a full-size ATX motherboard in here) and offers enough room for budget-oriented gaming builds. It has room inside for up to three 2.5-inch or 3.5-inch drives, graphics cards up to 315mm long and CPU coolers up to 155mm tall. It even has room for a DVD-drive if you somehow still need one.
You shouldn't expect too much from the materials and build quality as far as luxury goes, but with painted SPCC steel and dark plastics it should last. All things considered, Thermaltake's Versa H15 is a great budget case and also fitting for new PC builders.