codigo

<button id="roll" onclick="clickevent()">Rolagem Aleatória</button>

<p id="myNum"></p>

<table id="myTable" border="1">

  <tr>

    <th>Moeda/Minério</th>

    <th>Valor Base</th>

    <th>Modificador</th>

    <th>Cotação</th>

  </tr>

  <tr>

    <td>Cobre (🟤)</td>

    <td>0.1</td>

    <td>-1</td>

    <td id="cobre"></td>

  </tr>

  <tr>

    <td>Prata (🟣)</td>

    <td>1</td>

    <td>0</td>

    <td id="prata"></td>

  </tr>

  <tr>

    <td>Ouro (🟡)</td>

    <td>10</td>

    <td>+1</td>

    <td id="ouro"></td>

  </tr>

  <tr>

    <td>Platina (⚪)</td>

    <td>50</td>

    <td>+2</td>

    <td id="platina"></td>

  </tr>

  <tr>

    <td>ARVA (🔵)</td>

    <td>250</td>

    <td>+3</td>

    <td id="arva"></td>

  </tr>

</table>

<table id="myStore" border="1">

  <tr>

    <th>Item</th>

    <th>Valor Padrão (em Ouro)</th>

    <th>Valor Atual (em Ouro)</th>

    <th>Pagar com:</th>

  </tr>

  <tr>

    <td>Anel de Proteção +1</td>

    <td id="anel-padrao">100</td>

    <td id="anel-atual"></td>

    <td><select id="anel-pagamento" onchange="changeevent()">

      <option value="cobre">Cobre (🟤)</option>

      <option value="prata">Prata (🟣)</option>

      <option value="ouro">Ouro (🟡)</option>

      <option value="platina">Platina (⚪)</option>

      <option value="arva">ARVA (🔵)</option></select></td>

  </tr>

<script>

function clickevent() {

  var rannum = Math.floor(Math.random() * 6) + 1;

  document.getElementById("myNum").innerHTML = rannum;

  var cobre = Math.max(0.1 + (-1 * rannum), 0);

  var prata = Math.max(1 + (0 * rannum), 0);

  var ouro = Math.max(10 + (+1 * rannum), 0);

  var platina = Math.max(50 + (+2 * rannum), 0);

  var arva = Math.max(250 + (+3 * rannum), 0);

  document.getElementById("cobre").innerHTML = cobre;

  document.getElementById("prata").innerHTML = prata;

  document.getElementById("ouro").innerHTML = ouro;

  document.getElementById("platina").innerHTML = platina;

  document.getElementById("arva").innerHTML = arva;

  var anel_padrao = document.getElementById("anel-padrao").innerHTML;

  var anel_atual = anel_padrao * ouro /10;

  document.getElementById("anel-atual").innerHTML = anel_atual.toFixed(2);

  var anel_pagamento = document.getElementById("anel-pagamento").value;

  switch(anel_pagamento) {

    case "cobre":

      var anel_preco = anel_padrao * cobre /10;

      break;

    case "prata":

      var anel_preco = anel_padrao * prata /10;

      break;

    case "ouro":

  var anel_preco = anel_padrao * ouro /10;

  break;

case "platina":

  var anel_preco = anel_padrao * platina /10;

  break;

case "arva":

  var anel_preco = anel_padrao * arva /10;

  break;  

}