[danik.js]

<form method="post" action="" id="productForm">
	<select name="product">
		<option value='product1_1' data-price="10">Яйцо</option>
		<option value='product1_2' data-price="20">Мясо</option>
		<option value='product1_3' data-price="30">Фарш</option>
		<option value='product1_4' data-price="40">Молоко</option>
	</select>
	<label>Количество</label>
	<input type="text" value="100" name="quantity" size="10" maxlength="4" autocomplete="off"><br />
	Сумма:<b style="color:green"><span id="price">0</span> руб</b><br>
	<input type="submit" value='Купить' class="submit">
	<script>
		var productForm = document.forms.productForm;
		productForm.oninput = productForm.onchange = function() {
			price.innerHTML = this.product[this.product.selectedIndex].getAttribute('data-price') * this.quantity.value;
		};
	</script>
</form>