急求一个Javascript解决方案,谢谢大家了!

[javaman22]

<form action="dopurchase.php" method="POST" name="form1">
<tr>
<td>
购买数量：<select name="number" id="number" onchange="totalaccount(document.form1.number.value);">
<option value=1>1</option>
<option value=2>2</option>
<option value=3>3</option>
<option value=4>4</option>
<option value=5>5</option>
<option value=6>6</option>
<option value=7>7</option>
<option value=8>8</option>
<option value=9>9</option>
<option value=10>10</option>
<option value=11>11</option>
<option value=12>12</option>
<option value=13>13</option>
<option value=14>14</option>
<option value=15>15</option>
</select>
</td>
</tr>
<tr>
<td>
道具单价：30 G币
</td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="购买">
</td>
</tr>
</form>

希望在用户选择道具数量后,页面上自动显示总共的金额,就是单价乘上用户所选的个数,该如何处理呢?

[seabell]

<script>
function totalaccount(myvalue)
{
document.all.myspan.innerHTML=myvalue*30;
}
</script>
<form action="dopurchase.php" method="POST" name="form1">
<tr>
<td>
购买数量：<select name="number" id="number" onchange="totalaccount(document.form1.number.value);">
<option value=1>1</option>
<option value=2>2</option>
<option value=3>3</option>
<option value=4>4</option>
<option value=5>5</option>
<option value=6>6</option>
<option value=7>7</option>
<option value=8>8</option>
<option value=9>9</option>
<option value=10>10</option>
<option value=11>11</option>
<option value=12>12</option>
<option value=13>13</option>
<option value=14>14</option>
<option value=15>15</option>
</select>
</td>
</tr>
<tr>
<td>
道具单价：30 G币
</td>
</tr>
<tr>
<td>
总价：<span id=myspan>30</span> G币
</td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="购买">
</td>
</tr>
</form>

[javaman22]

谢谢百合心! :)