If post has custom field then display css-class

You most probably ran into problems with escaping code and HTML from each other.

Try this (added line breaks just for better readability):

<i class="
    <?php 
         echo (get_post_meta(get_the_ID(), 'Preis', true) != '') ? 
          'fa fa-check' : 'fa fa-times';
    ?>
">

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