They are not the same, and even both are supposed to help with swelling problems, how they help is very different..
Diabetic socks are nonbinding, soft, and supposed to be without seams (which can irritate the feet and cause/worsen sores in diabetics who have nueropathy/poor circulation/poor glucose control) or, at least, specialy designs seams. Many diabetics also have edema in the feet and legs, so regular socks tend to pinch terribly around the cuff top. Good diabetic socks should also allow the feet to breathe (prevent moisture from leading to funus/bacteria problems).
Support socks are used to prevent swelling in people who have poor circulation (elderly, heart patients, diabetics) - BUT - you should always check with your doctor first. They push the edemic liquids back into the blood/lympth system and can cause increased difficulties with the heart/lung/kidneys in some patients. They can aggrivate/result in sores as well (the last thing a diabetic needs). They can be a little difficult to put on, and your skin SCREAMS when you take them off (trust me on that). If you use them, always get the lowest level of compression that you need.
Anastaci - they are expensive because they are made of special design and materials, and they last quite a while is washed/worn properly. They really are different from regular socks. Diabetics often have foot problems, including edema. Regular socks hurt (they aren't as soft, stretchy, and thick as people think, can cause overdrying of the skin, and wear out fairly fast). I love my diabetic socks and I've addicted my whole family to them. Support socks are even more specialized and made of expensive materials (but they also last for years if you take care of them properly - I have some that are over 10 years old, which I still wear on occassion, like when flying across the country - they can help prevent embolisms, which can be a problem in many airline passengers). Don't assume everything is a rip-off - lots of things are well worth the money if you need them.