php - How to toggle elements with javascript

Solution:

I would write them both to DOM and usetoggleClass() from jQuery

<?php
$incl = $product->get_price_including_tax();?>
<div >
  <p id="excl" ><?php echo $product->get_price_excluding_tax();?> excl.</p>
  <p id="incl" ><?php echo $incl;?> excl.</p>
</div>
<button type="button">Click Me!</button>
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With following CSS:

.mycont > .price.incl {display: none}
.mycont.show_incl > .price.incl {display: block}
.mycont.show_incl > .price.excl {display: none}
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And jQuery:

$('button').on('click', function(){
    $('.mycont').toggleClass('show_incl');
});
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Fiddle Here

If you want to do it in pure javascript, I would suggest to call a functiononClick event on button, and toggle class like stated here