well you will want to consider airflow. as long as your cheap case has a good number of fans you will be fine. Obviously more expensive cases are made better and better materials, may have better cable management and other features. But if its just going under you're desk never to be seen a cheap case with a few fans should be fine. espeacily if budget is tight
Plus you wont feel as nervous about modding a cheap case,
you can hack a hole or two for more fans etc in a $25 case with no worries, its a little more stress-inducing on a $150 case hehe
if you find one you like post a link
Cons: Cheap build quality (you get what you pay for), misaligned screw holes, pointed edges that more expensive cases don't have, poor looks (have you ever seen a bada** case for $25? Me neither ), harder to install drives, and especially cheap fans. Terrible airflow. I have bought a case that came with only one exhast fan... that was DOA!
If I were you, dip a bit more moolah into a case. The upgradability will save you time, money, plus you won't regret it.
My best guess would be dipping at least $60 into a computer case. Seeing as you may only want to dip very little, I would like to give you some advice: research! A $25 dollar case would last you maybe a couple of years IF AT THAT. You'll save more money dipping into a $60 case that would last you maybe 5-6 years instead of dipping $25 into a bottom line case with little features.
I would recommend this case: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
For 60 bucks you get a toolless design with more upgradablility than cheaper cases on the market, plus a sleek look. Thousands of neweggers have reviewed this case. Out of around 2,500 reviews, almost 75% are 5 eggs.
Please take a second thought on how much you spend on a case.