Navigate to vendor/magento/module-catalog/view/frontend/templates/product/list/toolbar/sorter.phtml 

Find this code 

 <select id="sorter" data-role="sorter" class="sorter-options selectpicker">
 <?php foreach ($block->getAvailableOrders() as $_key => $_order): ?>
 <option value="<?php /* @escapeNotVerified */ echo $_key; ?>"
 <?php if ($block->isOrderCurrent($_key)): ?>
 selected="selected"
 <?php endif; ?>
 >
 <?php echo $block->escapeHtml(__($_order == "Position" ? 'Relevance': $_order)) ?>
 </option>
 <?php endforeach; ?>
 </select>

and replace with this code 

 

 <select id="sorter" data-role="sorter" class="sorter-options selectpicker">
 <?php foreach ($block->getAvailableOrders() as $_key => $_order): ?>
 <?php if($_order != 'Price'): ?> 
 <option value="<?php /* @escapeNotVerified */ echo $_key; ?>"
 <?php if ($block->isOrderCurrent($_key)): ?>
 selected="selected"
 <?php endif; ?>
 >
 <?php echo $block->escapeHtml(__($_order == "Position" ? 'Relevance': $_order)) ?>
 </option>
 <?php endif; ?> 
 <?php endforeach; ?>
 <option class="low-high">Price - Low to High</option> 
 <option class="high-low">Price - High to Low</option> 
 </select>

Now place this script at the end of sorter.phtml file 

 <script>
 jQuery(document).ready(function(){
 jQuery(".low-high").click(function(){
 window.location="?product_list_order=price&product_list_dir=asc";
 });
 
 jQuery(".high-low").click(function(){
 window.location="?product_list_order=price&product_list_dir=desc";
 });
 })
 </script>


Note: It will be better if you don't make the changes in core file. Copy the file from `vendor/magento/module-catalog/view/frontend/templates/product/list/toolbar.phtml` 
and paste into `app/design/frontend/Magenot/Your_Theme/Magento_Catalog/templates/product/list/toolbar/sorter.phtml`