Change input value onclick button - pure javascript or jQuery

Question

I have two buttons and if I click on some button I need to change value in input text and change total price (product price * value of button - 2 or 4 Qty).

I know

Answer 1

And here is the non jQuery answer.

Fiddle: http://jsfiddle.net/J7m7m/7/

function changeText(value) {
     document.getElementById('count').value = 500 * value;   
}

HTML slight modification:

Product price: $500


Total price: $500






Total 

EDIT: It is very clear that this is a non-desired way as pointed out below (I had it coming). So in essence, this is how you would do it in plain old javascript. Most people would suggest you to use jQuery (other answer has the jQuery version) for good reason.