Here's how I look at it.
Let's say you buy a pair of shoes for 80 bucks, which typically means it's a decent pair (not always, but usually) and you can get somewhere between 400 and 500 miles on the suckers. Now, let's say you run 80 miles a week, which you probably aren't if you're a grad student and looking for cheap shoes. This 80 dollar pair of shoes is going to last you about 5 or 6 weeks on that kind of mileage, so that's between about 14 and 16 bucks a week that you are paying to run on those shoes. If you're working at $8 per hour, which you're probably not, then 2 hours out of your 40 hours per week of full time work, or 5% of you're earnings, goes to staying in fresh, safe shoes and running injury free. Personally, I would pay way more than 16 bucks a week to stay injury free, and remember, that's if you buy expensive shoes that don't last too long and run them into the ground. You'll probably be spending under 10 bucks a week on shoes, and that's probably less than an hour's work for a grad student. Spring for the good stuff, your feet will thank you.