using html5 data attribute...
try this
Html
Product price: $<span id="product_price">500</span>
<br>Total price: $500
<br>
<input type="button" data-quantity="2" value="2
Qty">
<input type="button" data-quantity="4" class="mnozstvi_sleva" value="4
Qty">
<br>Total
<input type="text" id="count" value="1">
JS
$(function(){
$('input:button').click(function () {
$('#count').val($(this).data('quantity') * $('#product_price').text());
});
});