Wow! What software are you using to submit to them all at once? That would be a huge help! Photography has become so cheapened these days. I have had images on many many book covers over the years, and it was not unusual to make over $1,000 for a book cover. Even with my regular Getty stock, that has been drastically reduced these days.
And what I get from the micros is ridiculously low. But that is just a bit extra, as you say. I find that I have certain photos that are daily sellers though, and that helps because it does add up.
Good for you being a student and making some money on stock on the side. You should do well on here too - better than microstock by far!
I'm with most of the micros too now. I find that my best sales are through Fotolia/Adobe, Shutterstock and istock.
The software is called Stocksubmitter, it is free for up to 33 per month per agency, for more you need to pay, but 10€ for 250 photos is very reasonable. In case you have a mac, need to use the Windows version on Windows since the Mac version is not working really well. I might do a post about stock photography soon where I will recommend this software, it does safe a lot of time since you can also do all the tagging and categorizing within the software.
My photos are by far selling best on Shutterstock, also my earnings from Fotolia/Adobe Stock have been growing recently. Istock is on place three, but Alamy is catching up.