Basically, the adapter would require an extra element to bend the light correctly onto the sensor, this will increase distortion of the image (which already has a shitload of distortion from that lens) and will also likely make the entire thing look pretty soft focused.
The adapter will cost more because it includes this adapter, and isn't just a piece of metal like that you would find from a pentax or a nikon, which is just a metal ring and nothing else.
As for impossible? It's a minolta A mount. the A mount is 'gelded' which means it has no aperture ring and therefore you have absolutely ZERO control over the aperture of the lens - similar to canon, the aperture is selected electronically. You don't have a ring to change anything. So, you're going to have to shoot wide open all the time, which on this lens, means you're going to get a pretty crap, and soft focused image at every focal length.
lastly, it's a probably bad lens as it is, being it's probably a sigma kit lens, and doesn't even offer you any cool focal lengths or speed. 28-80 is effectively like 45-120, which really doesn't do a whole lot for you, it's most likely slow as hell and won't be very useful in low light.
You can find a used Canon 28-80 f3.5-f5.6 for around 30-40$ that will not only be sharper and give you autofocus, but it will cost about the same as getting an adapter for a borderline useless lens.