... di aver ben capito:

codice:
<form>
Prezzi 1

<select name="prezzi1" onChange="document.getElementById('risultato').innerHTML=parseInt(this[this.selectedIndex].value)+parseInt(this.form.prezzi2[this.form.prezzi2.selectedIndex].value);">
<option value="50">50</option>
<option value="150">150</option>
<option value="200">200</option>
</select>



Prezzi 2

<select name="prezzi2" onChange="document.getElementById('risultato').innerHTML=parseInt(this[this.selectedIndex].value)+parseInt(this.form.prezzi1[this.form.prezzi1.selectedIndex].value);">
<option value="250">250</option>
<option value="300">300</option>
<option value="350">350</option>
</select>


Risultato:
<div id="risultato"></div>
</form>
Fammi sapere !