Klaviyo does support Liquid, to calculate the amount a person would save with a coupon, you can use the following Liquid code:
{% assign discount_percent = 10 %}
{% assign sub_total = event.extra.SubTotal %}
{% assign discount_amount = sub_total * discount_percent / 100 %}
{% assign total_with_discount = sub_total - discount_amount %}
You saved {{discount_amount | currency}} with your {{discount_percent}}% discount!
Your new total is {{total_with_discount | currency}}.
This code sets the discount percentage to 10%, retrieves the subtotal from the event.extra object, calculates the discount amount, and subtracts it from the subtotal to get the new total with the discount applied.
You can customize the discount percentage and currency formatting to suit your needs.