Well if we go by the example given to us by oxana_kovelenko (+1) we can assume it's based on official currency rates which may vary depending on the global forex markets
As of time of writing 5EUR equals 5.87863USD but can basically vary per second
Usually banks will take the average rate from a timeperiod at the beginning of opening of the global forex market (e.g. a two hour period on mondays.) and apply that average as rate throughout the week. I can only assume Steam does something similar.
I also heard the GBP has a nice conversion rate atm, so it might be worth wild checking that out too.
5 USD equals 5 USD =4.25318EUR at time of writing, so i guess it's more or less that amount.
My source for checking actual rates and conversions has always been xe.com ,you can check any rate and convert currency quickly over there.