I would always endorse the idea of having at least one fast prime like that...if you can find one that gets you in the door for under $100 or so, that would be exactly what I'd recommend to start with. If you find you use it a lot, and really come to love low light shooting with it, but find it a bit too long, you can always look to invest in a good 28mm, 30mm, or 35mm version down the road. But a cheap 50mm is always a great place to start. I myself got a Minolta 50mm F1.7 as my first low light lens, picked up used for $60. I'd suggest the same - you might even find that lens used for even less...check out KEH.com to see if they've got any.
It is worth having in my opinion. If there is a 35mm for less than $100 more though, then I would go for that instead. I don't know Canon enough to tell you.
The Canon 50mm f/1.8 will run you between $100 and $130 depending on where you look online.
I've used the 50mm f/1.4 and just absolutely LOVE that lens and was very impressed with how well it did on dark rides / shows. But that lens will run you ~$450.
Here are some others on Canon (these are only the ones I know, I'm sure there are others)......
Sigma 30mm f/1.4 ($489 on Amazon)
Canon 28mm f/1.8 ($800 on Amazon)
Sigma 28mm f/1.8 ($450 on Amazon)
Canon 35mm f/2 ($399 on Amazon)
Canon 35mm f/1.4 ($1479 on Amazon)
Hey DoleWhipDVC........
I know you just got the Tamron 10-24.......did you know that Tamron is offering a $50 rebate on that? I just got the ad from Calumet Photo. Looks like it applies for any purchase from May1 - July 31st.
Makes the Tamron more inticing.....
Yeah, after just buying the Tamron 10-24, I'll need to stay in that "lower price" bracket for awhile! But I'm going to start looking for a good prime in that 100-150 price range. Thanks for your advice!