Simple Javascript Form

I have a very simple issue that I need help with...

I have a form that I want a dynamic submit button based on the values entered in a couple of select inputs.

Here is my script:

<script type="text/javascript" language="javascript">
function getPrice () {
var quantity = document.getElementById('quantity').value;
var ad_length = document.getElementById('ad_length').value;
if (ad_length == 1) {
amount = 5.99;
total = document.write('The Discounted Price For ' + quantity + ' Ads of ' + ad_length + ' Months Is $ ' + eval(quantity*amount));
}
else if (ad_length == 2) {
amount = 7.49;
total = document.write('The Discounted Price For ' + quantity + ' Ads of ' + ad_length + ' Months Is $ ' + eval(quantity*amount));
}
else if (ad_length == 3) {amount = 12.49;
total = document.write('The Discounted Price For ' + quantity + ' Ads of ' + ad_length + ' Months Is $ ' + eval(quantity*amount));
}
else if (ad_length == 6) {amount = 14.99;
total = document.write('The Discounted Price For ' + quantity + ' Ads of ' + ad_length + ' Months Is $ ' + eval(quantity*amount));
}
else if (ad_length == 12) {amount = 22.49;
total = document.write('The Discounted Price For ' + quantity + ' Ads of ' + ad_length + ' Months Is $ ' + eval(quantity*amount));
}
}
</script>

Select #1

<select size="1" name="HowMany" id="quantity" onChange="window.document.form.B1.value=getPrice('total');">
<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>
<option value="16">16</option>
<option value="17">17</option>
<option value="18">18</option>
<option value="19">19</option>
<option value="20">20+</option>
</select>

Select #2

<select size="1" name="Length" id="ad_length">
<option value="1">1-Month</option>
<option value="2">2-Months</option>
<option value="3">3-Months</option>
<option value="4">4-Months</option>
<option value="6">6-Months</option>
<option value="9">9-Months</option>
<option value="12">12-Months</option>
</select>

And here is my submit button

<input type="submit" name="B1" value="" style="background:url(images/prototype.png); background-repeat:no-repeat; width:446px; height:48px; border:none">

Any help you can provide will be greatly appreciated.

 

That's rather vague; exactly what do you mean?
document.write can only be used a build time, not after the document is loaded.