Deleted

Chances of winning:

1/number of entries in the giveaway you entered. I'M A GENIUS

"Take the number of giveaways (while also being mindful of multiple copies) and divide by the sum of all the entries in said giveaways."
Wrong, the formula isn't that easy. I don't know how to explain it in English though.
For example you enter 3 giveaways, each has 3 entie. With your formula your chance to win would be 100% but in reality you can loose all giveaways.

Had a bit of spare time after reading your post. When you're using Google Chrome, create a new text document, name it whateveryoulike.user.js (the user.js is important, also check that you really renamed it that way and didn't forget to delete ".txt") and copy the following code in it:

// ==UserScript==
// @name Steamgifts probabilities
// @description
// @version 1.0
// @namespace 0x442E472E
// @author 0x442E472E
// @run-at document-end
// @include http://www.steamgifts.com/giveaway/
// ==/UserScript==
if(window.top == window) {
var load,execute,loadAndExecute;load=function(a,b,c){var d;d=document.createElement("script"),d.setAttribute("src",a),b!=null&&d.addEventListener("load",b),c!=null&&d.addEventListener("error",c),document.body.appendChild(d);return d},execute=function(a){var b,c;typeof a=="function"?b="("+a+")();":b=a,c=document.createElement("script"),c.textContent=b,document.body.appendChild(c);return c},loadAndExecute=function(a,b){return load(a,function(){return execute(b)})};
loadAndExecute("//ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js", function() {
var entries = parseInt($('.rounded.entries').html().split(" ")[0].replace(/,/g, ""));
var regexp = $('h1.title').html().match(/((\d+) .+)/);
var copies = regexp != null && regexp.length >= 2?parseInt(regexp[regexp.length - 1]):1;
$('.rounded.entries').html($('.rounded.entries').html() + " "+ (parseInt(((copies/entries)1000000))/10000)+"%");
});
}

Save the file, open the extension tab in Chrome and drag&drop the file into it. You should be asked to install that script. That's it. It shows the probability at the giveaway detail page (not "featured" at the top of the page and also not at the quickview of the enhancement addon). And please no complaints about the code, i wrote it only quickndirty :P

inb4
"I LOOKED WHEN I ENTERED AND IT SAID 100% CHANCE TO WIN, BUT I DIDNT WIN THIS SITE IS A SCAM"