Javascript question. Auto calculate form fields

This does not involve Wordpress I don't think but I need help.

I have a form with two fields: Price and Quantity

I would like to display dynamically the total based on the values of Price and Quantity.

When the input to those fields are not Floats, just show N/A on the Total field. Ideally, the Total Field is not a form input readonly though and it's a span or <p> tag of some sort.

Can someone please help? I've googled the hell out of it for like 3 hours and no luck.

<table>
<thead>
<tr>
<th>Each Gift</th><th style="margin-left:10px;">Quantity</th><th style="margin-left:10px;">Total gift amount</th>
</tr>
</thead>
<tbody>
<tr>
<td>'.$mp->format_currency().'&nbsp;<input onkeyup="return ourhm_UpdateTotal(this,_mp_inventory)" class="toAdd" type="text" name="_mp_price" value="'.$price.'">
</td>
<td>
<input class="toAdd" onkeyup="return ourhm_UpdateTotal(this,_mp_price)" style="margin-left:10px;" type="text" name="_mp_inventory" value="'.$inventory.'">
</td>
<td>
<input readonly="readonly" id="totalgift" style="margin-left:10px;" type="text" name="totalgift" value="">
</td>
</tr>
</tbody>
</table>