Coolstuffinc.com is a good source for singles. Most are .39 or .49 cents, can be more depending on rarity.
The exception to this are some of the dice from the starters, particularly Avengers Vs. Xmen. People don't get duplicates of the commons in the starter pack and the other cards in the boosters are usually more rare.
A few Examples:
Human torch: 1.99