Mail merge - format as currency or as text

[macropod]

I'm doing the Alt+F9 bit, but I don't understand what you mean by "your datasource record's field name"

The procedure refers to Ctrl-F9, not Alt-F9, which won't do the same thing.
As for the field name, in your first post you said:

I have a mail merge that comes in from Excel, where one value can either be a price eg £400.00, or can be a statement, which is "TBA (£515 per day)".

You should use whatever the name of that field is instead of 'Data' in the field code.