[xAockd]

<form onchange="calc()">
    <input type="radio" name="design" value="300"/>100шт 350 руб
    <input type="radio" name="design" value="600"/>200шт 600 руб
    <input type="radio" name="design" value="850"/>300шт 850 руб
</form>

var radio = $("input[name=design]:checked").val();