In most situations it is fairly straight forward to format a control to show currency, you change the appropriate property e.g. Text to include the Text function such as:
Text(ThisItem.Price,"[$-en-US]£#,###.00")
However, to do this with a currency field in a Form Datacard takes a bit more effort. To do this you will want to edit the data card and add a new Label on top of the DataCardValue field. The new label needs to obscure the text of the DataCardValue – one way to do this is to set the fill on the label to the same colour as the DataCard.
Once this has been done, you can set the Text property of the label to:
Text(Parent.Default,"[$-en-US]£#,###.00")
Note: In some circumstances you may need to perform some additional modification of the data:
Text(Value(Parent.Default),"[$-en-US]£#,###.00")