IMHO, saving the value in a worksheet is by far the easiest and best approach.
The ability to do that is one of the nice things about programming in Excel. You
could save  the exchange  rate on another sheet which you can set  to xlVeryHidden
and password protect.
- I don't recommend having a user  open the VBE and edit code (even if it's
just a Constant). - Another option is to write the data out to another file,
such as a text file or another Excel workbook. The next time  your application
workbook is opened, your code can look for the other file to retrieve the
exchange rate. Hopefully the other file will still be there.