Using javascript to get value of textbox

I have a form that uses a bunch of radio buttons, where I give each radio button a value. The radio button that is marked other has a textbox next to it and we need that amount as the value of the radio button. How do we do this?
here is a piece of the sampled code:
<input name="Ecom_Cost_Total" type="radio" class="donate_check" value="50" />
<div class="donate_title">Friend </div>
<div class="donate_amount"> $50</div>
</div>
<div class="donation_option">
<input name="Ecom_Cost_Total" type="radio" class="donate_check" id="other" value="other" />
<div class="donate_title"> Other </div>
<div class="donate_amount">
<input name="donate_amount_us" type="text" id="donate_amount_us" size="5" maxlength="5" /></div>
The form is taking the values frm "Ecom_Cost_Total" and is then saying that the amount is invalid.
Help?