well, yea, some of the 'equivalents' are cheaper i guess :think:
And it depends on the mount. Sigma builds both Nikon mounts and Canon mounts. If you use a Nikon camera, you need to buy the lens that was manufactured with a Nikon mount.
Sigma has lenses for most camera brands. There's also other 3rd party lenses from brands e.g. Tamron, Tokina etc. Personally I have both Sigma and Tamron which works quite well for my level.
yes cheaper and they are compatible with both nikon and canon.
to explain more: if that sigma lens is made for nikon mount, then only nikon body can use it, not canon body, vice versa.