Denominations for Physical Bitcoin Scratch Cards

[Mageant]

I'm planning on making scratch cards loaded with Bitcoins (mBTC).

They will be similar to prepaid mobile phone cards.

The private key will be hidden under the scratch panel. So you could theoretically reuse them as long as you don't scratch that open.

The idea is that these are cheap and only hold a small amount of Bitcoins to discourage counterfeiting.

You can use them for small in-person purchase, or just a kind of small present or tip you can give to people.

Also, the idea is (potentially) for use in developing countries where people have little money and possibly not even electricity.

What different denominations should there be (all numbers in milliBitcoin)?