How to make option selection change button link?

I have a drop down list and a button. I want to make the selection of list reflects on the button link.

Example JSFiddle

Here is the example code:

<select style="float:left;">
<option value="5">$5.00 monthly</option>
<option value="10">$10.00 monthly</option>
<option value="15">$15.00 monthly</option>
<option value="20">$20.00 monthly</option>
</select>

<div id="button" style="width:100px;height:30px;float:left;background:#ccc;line-height:30px;text-align:center;margin:auto 5px;">
<a href="http://ordernow/5/">Order</a>
</div>

What I want to achieve is when the user for example selects the second option, the button link changes to http://ordernow/10/ instead of http://ordernow/5/ and so on. Can someone inform me how do that or provide a link for a tutorial regarding this? Thank you.