How much are you looking to do there? If you want to get "boots on the ground" and check it off the list, the King George Island options could be good, and more affordable. Victory Adventure Expeditions (
http://www.victory-cruises.com/fly_a...ca.voyage.html) offers one opportunity, with both a day trip and an overnight option. You'll see penguins, so that's cool, but I have not used them so cannot attest to the company's quality.
If you want to see some serious Antarctica and do something intense, I strongly recommend Adventure Network International, which runs a camp at Union Glacier (
http://www.adventure-network.com/union-glacier-camp). It's at 80 degrees south, which is well inland and lets you see the Antarctica you saw in National Geographic as a kid. I was there for the Antarctic Ice Marathon (if you're a runner, that's the best way to get all 7 continents done) and they also have visits to the South Pole and a few days' trip to a penguin colony. ANI's trips are more expensive, but well worth it -- you get a few days of living on the continent and it can be a life-changing experience.
Have fun, whatever you do!!