Prime Lens For Canon T2i

In that price range I'd recommend Canon's  50mm 1.8 lens.  Often refered to as Canon's  'plastic fantastic' because of its lightweight construction.  But for an inexpensive lems it delivers good results and the wide open aperature can give you some great shallow focus shots. This would be my first choice for a lens under $100. I've seen it priced from $89 to $129.

Nikor lens for T2i. Another way to go is to buy a used Nikor lens and an inexpensive Nikon to Canon adaptor.  For under $100 you can get a very sharp piece of glass that has a manual aperture ring on the lens. I purchased a vintage 50mm 1.4 for around 70 bucks; it's sharp but very warm– it has a look I love. Since you mentioned "prime" lens I thought immediately of video use. This solution won't give you auto focus or any iris control. Try Keh.com

I'll second joey-j's suggestion about buying a used prime lens and suggest an Olympus OM 50mm f1.8. It's a solid and fast lens with a manual aperature ring that you can find at keh.com right now for $49. Pair it with a Fotodiox Canon EOS to Olympus OM lens mount adapter for $15.95 from Amazon and you'll have a nice prime lens for your T2i well under $100. I had an old Olympus film camera with a 50mm f1.8 lens and purchased the adapter to use it on my T2i and it's sweet. BTW, many DSLR videographers on a budget call the 50mm f1.8 lens the "nifty 50" because they can shoot all of their footage with it in almost any kind of lighting situation. Good luck!