[Areso]

<script>
   function Calc()
{
   
   var priceitem = Number(document.getElementById('priceitem').value)
    
   alert (priceitem)
    
   var priceenchant = Number(document.getElementById('priceenchant').value)

   alert (priceenchant)
    
   var basicchance = Number(document.getElementById('basicchance').value)
   
   alert (basicchance)
    
   var safetyenchant = Number(document.getElementById('safetyenchant').value)
   
   alert (safetyenchant)
    
   var levelenchant = Number(document.getElementById('levelenchant').value)
   
   alert (levelenchant)
    
   var sum=0
   var sumpriceitem = 0
   
  
  if (levelenchant >= 0) and (levelenchant=<20)
    {
        if (basicchance >=1) and (basicchance=<100)
        {
            if (levelenchant > safetyenchant)
            {
                for (i=1; levelenchat-safetyenchant; i++)
		{
                sum = sum + (1 - (basicchance / 100) ^ i) * priceitem
		}
            }
            sumpriceitem = priceitem + priceenchant * levelenchant + sum
	    document.getElementById('summary').value = sumpriceitem
        }
        else
	{
        alert=("Chance of successful enchant incorrect. Please, re-enter chance of enchant from 0 to 100")
	}
    }

    else
   {
   alert=("Level of enchant incorrect. Please, re-enter level of enchant from 0 to 20")
   }
}
</script>

<textarea id="priceitem" rows="1" value="100"></textarea>
<p>
<textarea id="priceenchant" rows="1" value="5"></textarea>
<p>
<textarea id="basicchance" rows="1" value="70"></textarea>
<p>
<textarea id="safetyenchant" rows="1" value="3"></textarea>
<p>
<textarea id="levelenchant" rows="1" value="4"></textarea>
<hr>
<textarea id="summary" rows="1" readonly="1"></textarea>
     <p>
    <button onclick="Calc()">Calc</button>

И, почему-то, не вгоняет значения по умолчанию в текстовые поля.