You want cheap and fast.... these are the options
The telephotos:
1)70-200/2.8 zoom... look at the sigma HSM too. It is a fast lens but not that as fast as the other primes.. for your dance performance, it is THE lens to use if you can get decent shutter speeds. You can zoom in and out.
2)85/1.8 USM... one of the best bang for the buck lens. Fast AF, Fast Aperture, nice Bokeh and perfect focal length for portraits. For your dance performance, depending where you are standing, this can either mean headshots or half body
3)100/2 USM... not that popular because it is more expensive than the 85, slighlty slower and the focal length is almost the same.
4) 135/2.8 Soft focus. If you are really on a budget and can find a cheap one second hand, this is one of the best lens to get for the least amount of $$$. Just dont use the soft focus feature
5) 135/2L... same thoughts as with the 85mm. Perfect focal length, fast AF, fast aperture. But point to note, i have no idea what kind of dance you are shooting and if you want to get motion stopping shutter speed. From experience anywhere between 1/125 to 1/250 will give me action sharp photos if the dancers are not dancing that fast or holding a pose. 1/500 is the shutter speed i try to get if i want no blurring in the hands or legs... SO if you feel you can get away with speed like 1/100, the 85mm is actually a better lens than the 135/2L. Why? first, at 1/100, you can be sure that you have no hand shake while with the 135, you have be more careful. and the 85 is slighly faster..
6) 85/1.2L.... i think you are better off with the 85/1.8 or 135 ( I have the 85L and its AF is very very slow... not suitable for low light stuff unless you are prepared to MF)
7) 200/1.8L.... this lens is very very expensive... it demands a monopod to handhold for long periods of time and you need faster shutter speed to hand hold.
WIde and Normal options
1)50/1.4 and 50/1.8... i dont like the 50mm focal length and i think you are better off with the 85/1.8.
2) 35/2... never used it...
3) 28/1.8USM. I have this lens and it works very well on a 1.3x crop (35mm). Dont know if it is wide enough on a 1.6x crop. Fast AF Fast aperture and good focal length (on 1DmkII). Perfect for your low light dance performance.
4) 20/1.8 Sigma. I never really used this lens but i know a couple of people who used this pretty often for low light dance performance. The Quality variation is very high and you might get a good copy and you might not. So buyer beware.
5) 35/1.4L.... if you have a 5D or 1DmkII, i think the focal length is pretty good... expensive but this and the 24/1.4L is king of wide angle low light
6) 24/1.4L ... see point 5
* for the wide angle shots, they are a must if you are standing near the stage. the 50 and 85 are way too long for full body or gp shots. Personally, I would like to hold 2 bodies with fast primes for such concerts if i want to cover all grounds.
** For what you are shooting I use a 1DmkII with 24-70L(standing close) or 70-200 IS (standing far away). IF the f2.8 is not fast enough... i pull out the 28/1.8 (on 1DmkII)and 85L or 100/2(on 300D).
