Hello Hamsterlover, welcome to the Hamster Central forum!
I find that in the past the best way to convince my mother was to get her when she was PMSing and offered to buy her Cinnabons from the mall of she let me have the animal lol, and its worked in the past lol...
Anyhow.. Another way I would go about doing this is making a list of all the reasons why I should have one and also a list of things that I have done that show I am responsible. I also would read up as much as I could about the animal so I could impress my mother buy showing her how much I had read and learned as well as showing her how much I am really thinking about it and want it.
If your in college you give the whole talk about how you arent a child anymore and that you are an adult and she shouldnt treat you as such anymore, then youll probably get from her the line about " so long as your under my roof!" blah blah blah lol..
If you are going to get a hamster I suggest you do get a job on the side to pay for it, especially in the event it needs a vet, it certainly doesnt sound like your mother would be too happy to pay for more vet bills, and being in college you are old enough to take that kind of responsibility yourself.
Anyway there are some of my techniques of the past.. I still use them on my hubby hehe.. Well besides the PMS thing.
Good Luck!