When entering options within the Product Options app, non-alphanumeric characters are ignored. You can force them in by using HTML Codes for the symbol you want to display.
For example: If the value needed for the option is -4.00, enter −4.00 as the option value.
Please refer to Set up Options and Relationships in Product Options 2.0 for the steps.
Note: This is not to be confused with subtracting from the cost. There is no ability to reduce a price through Shopify’s Variants or Product Options.