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A simple way how to display net and gross prices on the product page and in the category:

Theme classic
Category page:
/themes/classic/templates/catalog/_partials/minatures/product.tpl
Product page:
/themes/classic/templates/catalog/_partials/product-prices.tpl

Replace:

{if '' !== $smarty.capture.custom_price}
   {$smarty.capture.custom_price nofilter}
{else}
   {$product.price}
{/if}

With:

{if '' !== $smarty.capture.custom_price}
  {$smarty.capture.custom_price nofilter}
{else}
   <p class="your-class">{Context::getContext()->getCurrentLocale()->formatPrice($product.price_tax_exc, $currency.iso_code)}<br />
   <span class="your-class-2"> {l s='tax excl.' d='Shop.Theme.Catalog'}</span></p>
   {$product.price}
{/if}

A simple way how to display net and gross prices on the product page and in the category:

Theme classic
Category page:
/themes/classic/templates/catalog/_partials/minatures/product.tpl
Product page:
/themes/classic/templates/catalog/_partials/product-prices.tpl

Replace:

{if '' !== $smarty.capture.custom_price}
   {$smarty.capture.custom_price nofilter}
{else}
   {$product.price}
{/if}

With:

{if '' !== $smarty.capture.custom_price}
  {$smarty.capture.custom_price nofilter}
{else}
   <p class="your-class">{Context::getContext()->getCurrentLocale()->formatPrice($product.price_tax_exc, $currency.iso_code)}<br />
   <span class="your-class-2"> {l s='tax excl.' d='Shop.Theme.Catalog'}</span></p>
   {$product.price}
{/if}