WP7 Json serializer doesn't handle "space"?
Certain European cultures use spaces instead of commas for large numbers, so try using the appropriate CultureInfo before you parse the string:
CultureInfo ci = new CultureInfo("fr-FR");double d = double.Parse("1 000", ci); // returns 1000.0
Try using the newtonsoft json serializer. http://json.codeplex.com/They have a binaries for wp7, and is better that the datacontractserializer (in my view)
I've just noticed that the second answer of your mentionned question on StackOverflow is the same as the one i'm talking about.
Use JsonConvert.Deserialize<T>(string json) with T as your result (ExchangeRate ?)