Full details over on the Windows blog. Here's the quick rundown:

Starter (typically only found on low-cost netbooks) to Home Premium: $79.99.
Home Premium to Professional: $89.99.
Home Premium to Ultimate: $139.99

Unlike Windows Vista, using Anytime Upgrade only takes about 10 minutes, and doesn't require a full system upgrade like with Windows Vista which could take anywhere up to a couple of hours.

The Family Pack, which includes 3 licenses for Home Premium is $149.99, over $200 cheaper than buying them separately, but it sounds like it's going to be for a limited time only.