It would be nice if all money fields like the one when entering a budget (/cgi-bin/koha/admin/aqbudgetperiods.pl?op=add_form) had a tool tip to show not to use the currency character.
I don't know if this is still an issue on some pages, but it would be better solved by using JS validation and/or <input type="number"> for entries. I'm closing this and asking that if anyone runs into this they file a bug report for the specific instance.