The credit was a currency used by many planets and empires during a very long time. Apparently, many different civilisations had different values.
The Stoish civilisation, for example, had credits which were stored on "Credit Cards'". The Doctor said that £1,000,000 was equal to 50,000,056 credits in the early 21st century, meaning that the average credit was roughly equivilent to 2 pence. (DW: Voyage of the Damned)
Credits was used in various Human Earth Empires. In the 42nd century, in the Second Great and Bountiful Human Empire, the price of an Ood slave was 50 credits. (DW: Planet of the Ood)
Credits was used at least until the year 200,000. At this point a Kronkburger onboard Satellite Five costed 2 credits 20. (DW: The Long Game)