if get_post_meta is empty do something

You could use the empty function inside your if as such :

<?php if( empty( get_post_meta( $post->ID, 'price_list_category1', true ) ) ) : ?>style="display:none;"<?php endif; ?>

The above returns an error, you should assign the return value to a variable. See my edit below.

Warning

empty might not be the best option depending on the values you store in the meta. Values like false, 0 etc… will be considered empty.

Check the PHP manual for the full list of values that are considered empty.

Edit

You can try assigning the meta to a variable, and using that in the if statement.

 <?php
      $price_list = get_post_meta( $post->ID, 'price_list_category1', true );
 ?>

And then…

 if( empty( $price_list) ) : ?>style="display:none"<?php endif; ?>