dynamic var for multiplication in javascript

Sorry, your whole idea on how to use/handle events is wrong. The parameters to the actual GetTotalPrice call comes from nowhere, they don't exist in the context of your call. I don't even want to think how you handle will execute when called like that. You can see it under the debugger, but there is no a big need to waste time on that as it will always fail.

First parameter which can really be passed to event handler is the event object, which has a reference to the event target. In your case, you don't need it, but this is certainly not the numeric value and not a DOM object:
https://developer.mozilla.org/en-US/docs/Web/Events/click[^],
https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent[^].

You don't even need it, so you can write an event handler without arguments. As to the arguments txtqty, txtprice, txtTotalPrice, you will have to pass it via the outer context. But you need to create a certain context for your event handler, so you would rather need to set up the event handler in code, not in the HTML element attribute:

var txteqty = document.getElementById("txteqty");

// ...

txteqty.onclick = function() { // or onkeyup, but it looks a bit weird to me
    var txtqty = // calculate if somehow, based on your code in the 
                 // outside context
    var txtprice = //... same thing
    var txtTotalPrice = //... same thing
    GetTotalPrice(txtqty, txtprice, txtTotalPrice); 
    // better call is "ShowTotalPrice", because you are not returning a value
    // ...
}

If, for example, your three arguments come from other elements, you can calculate the string values using document.getElementById("appropriateId").value and then parse it to the number object, for example: http://www.w3schools.com/jsref/jsref_parseint.asp[^].

—SA