For the first time in the history of the Gran Turismo franchise, you can now purchase (via pre-order) in-game credits with real-world currency. Credit packs went on sale earlier yesterday at Amazon.co.uk, available in the following denominations and prices:

Amazon.co.uk is apparently the first retailer selling the credit packs anywhere; expect the packs to go on sale in other denominations in other regions when the game is officially released worldwide on December 6th. Thanks to Mike for the tip!

Update –

Eurogamer has posted an article disclosing cost on the two remaining credit packs:

or these. They are as follows: