ocultar compo de formulario segun radiogroup

[Cap.Buscapina]

<form name="datos" method="post" action="pmp.php">
<input type="radio" name="trans" value="comprar" onclick="if(this.checked){document.getElementById( 'precio').style.visibility='visible';}"> Comprar
<input type="radio" name="trans" value="vender" onclick="if(this.checked){document.getElementById( 'precio').style.visibility='hidden';}"> Vender <br><br>
CANTIAD: <input type="text" name="q" size="4"><br><br>
<div id="precio">PRECIO: <input type="text" name="p" size="4"></div><br><br>
<input type="submit" value="Ejecutar">
</form>