My reference for the expected behavior is the old 8.x-1.9 version: I'm at work for an update of an old Drupal 8 site.
There I could for sure set up a rewrite pattern for a field containing a price value, to put a "€" character before the number. I had no problem in having that number multiplied by 0.8, I mean the Simple Math Field was taking the number itself, ignoring the rewriting, as if the rewrite rule affected just the way the field was displayed. Passing to Drupal 9 and to version 3.0.1 of the module, the result of the mentioned multiplication is 0 anyway, as long as the rewriting procedure of the source value is active.
The result from Simple Math Field should be 0.
Curiously enough the problem does not occur if some character is set as a suffix of the numeric value, even if the suffix directly follows the numeric value, that is with no blank space in the middle.
Reverting the behavior back to the behavior I could rely on with the old 8.x-1.9 version: I could safely set up a rewrite rule to see a prefix for a source field value and see Simple Math Field perform the requested operation on the value, as if the object of the operation was the value itself and not the string resulting from the rewriting. After all, why should Simple Math Field ever work on a value that likely is not a number anymore? Rewriting a value would likely produce a string, wouldn't it?
I take this chance to ask you whether using version 8.x-2.0-beta2 I would have the same problem mentioned above.
Thanks in advance!
Andrea
Active
3.0
Code
On D10 I tested using the build-in Files view. I added a Simple Math field to that view, selected Size as the field to use in the calculation, put 10 * @filesize in the Formula box, and put € in the Prefix box. It looks like it's doing the math correctly.
Could you test this out using the Files view and post screenshots of how you've set everything up?