For cheap and low light and portability - canon's 50mm f/1.8 "plastic fantastic" is a great lens. You can probably get it for less than $80 by poking around.
For a bit more range, Tokina's 28-70mm f/2.8 was a nice lens - esp. on a camera with a cropped sensor (like the 30d) - it looks like it's since been replaced by the 28-80mm f/2.8. I liked that lens better than the canon 28-105 f/4-5.6 simply because of the wider aperture. Sigma and Tokina also have similar lenses in this range and they're all in the $100-250ish range (between ebay/used and new). The nice thing about buying 3rd party lenses off of ebay is they don't hold their value like canon's lenses do - so they're frequently as much as half the cost of the lens new.
There's also a number of inexpensive 28-200mm and 28-300mm lenses - that perform less well in low light but are compact and portable. I don't own/have any of these - but there've been times i've considered getting one. There is a price paid in terms of quality with those lenses - but I think some of that gets masked by the smaller sensor (problems around the edges of the image being most frequent - and the cropped sensor gets rid of those edges :). The really nice thing about a 28-200 or a 28-300 lens is no need to change lenses for a wide range of situations - though, another drawback is they aren't as good with the low light as an f/1.8 or even f/2.8 lens would be.
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For cheap and low light and portability - canon's 50mm f/1.8 "plastic fantastic" is a great lens. You can probably get it for less than $80 by poking around.
For a bit more range, Tokina's 28-70mm f/2.8 was a nice lens - esp. on a camera with a cropped sensor (like the 30d) - it looks like it's since been replaced by the 28-80mm f/2.8. I liked that lens better than the canon 28-105 f/4-5.6 simply because of the wider aperture. Sigma and Tokina also have similar lenses in this range and they're all in the $100-250ish range (between ebay/used and new). The nice thing about buying 3rd party lenses off of ebay is they don't hold their value like canon's lenses do - so they're frequently as much as half the cost of the lens new.
There's also a number of inexpensive 28-200mm and 28-300mm lenses - that perform less well in low light but are compact and portable. I don't own/have any of these - but there've been times i've considered getting one. There is a price paid in terms of quality with those lenses - but I think some of that gets masked by the smaller sensor (problems around the edges of the image being most frequent - and the cropped sensor gets rid of those edges :). The really nice thing about a 28-200 or a 28-300 lens is no need to change lenses for a wide range of situations - though, another drawback is they aren't as good with the low light as an f/1.8 or even f/2.8 lens would be.