The list group already looks pretty decent. However, one nice modification would be if the purchase price for each item would stand out a bit more. To this end, we can use Bootstrap's badges. Badges provide a nice way to create content that stands out and catches the user's eye. To this end, we simply place the prices inside a span and apply the badge classes to it, along with selecting an appropriate context color:

<ul class="list-group">
     <a 
        href="#"
        class="list-group-item list-group-item-action active">
         Photo #123.A purchased for 
        <span class="badge badge-warning badge-pill">€23
        </span>
     </a>
     <a 
        href="#"
        class="list-group-item list-group-item-action">
         Photo #456.A purchased for 
        <span class="badge badge-warning badge-pill">€42</span>
     </a>
     <a href="#" class="list-group-item list-group-item-action">
     Photo #123.B purchased for 
        <span class="badge badge-warning badge-pill">€42</span>
     </a>
 </ul>

Note that badges support any of the nine context colors. Therefore, badge-warning can be replaced with badge-primary, badge-secondary, badge-danger, badge-info, badge-warning, badge-success, badge-light, or badge-dark.