What if you bought DAO Tokens at a rate higher than 1:100 during the initial funding period, and would now like to know how to receive a refund from the extrabalance?

At the hard-fork block #1,920,000 , the ETH balance from The DAO and it's child DAOs (which includes the balance from the extraBalance account) were transferred into the Withdrawal contract.

Immediately after the hard-fork, at block #1,920,090, the Withdrawal contact's trusteeWithdraw() function was called in this transaction to transfer 463,798.08818109 ETH into TheDAOCurator multisig wallet.

The current TheDAOCurator wallet balance is 463,859.030790534 ETH ($5,802,876.48).

While the method of refunding the amounts paid in excess of 1 ETH = 100 DAOs is still being decided, the curators are trying to work out the fairest way that The DAO investors can be refunded.

Quoting vladzamfir in reddit.com/r/ethereum - Hard Fork Specification — Slock.it Blog dated 15 July 2016:

We're returning it to the purchaser, not the current token hold.

If during the creation period you paid 1.5 eth for 100 dao tokens with address 0xdeadda0, and have since transferred the 100 dao tokens to me at address 0xbeefda0, I would be able to claim 1 eth for the 100 dao tokens (by sending them from 0xbeefda0), and you would be able to claim 0.5 eth by sending a message from 0xdeadda0.
A very ingenious method of refunding The DAO investors!

One question I have with this refund method is: How are The DAO investors who bought in the last two weeks of the crowdfunding through an exchange be refunded the excess amount, when the address recorded with the purchase is the exchange's address?

Quoting GrifffGreeen in reddit.com/r/ethereum - Curator Multisig: extraBalance & childDAO tokens announcement Early next week dated 21 July 2016:

Expect an update detailing how the people that sent ether to the extraBalance and childDAO token holders will be able to claim their ether early next week.

For the extraBalance: It will likely be a smart contract that will allow any one who paid into the extraBalance to claim the ether they sent to it, with or without DAO tokens.

The childDAO token holders can also join the #child_dao_community in The DAO's Slack to stay up to date on the developments.

As the excess amount over 1 ETH = 100 DAOs will be refunded to the original account, make sure that you have access to the original account and passphrase to unlock the account.

This answer will be updated when more details of the process to refund the excess amounts is announced.