Hello @onyx_au
Welcome to Shopify Community.
Okay, so if you just want to show the discounted price dynamically calculated it can be done via hardcoded.
{% if product.tags contains 'ABC' %}
{% assign discount_value = 10 %}
{% assign discount_amt = 100 | minus: discount_value %}
{% assign discounted_total = product.price | times: discount_amt | divided_by: 100 | round: 2 %}
{{ product.price | money }} <del>{{ discounted_total | money }}</del>
{% endif %}
Let me know if you need help in the setup.
Thanks