Zero! Upgrade your life, upgrade your fitness, upgrade your technique and skills, upgrade your diet, etc. Or, if you must buy something, get something that you can really feel, like high-quality tires, get a nice pump, get some clothing, gloves, or whatever. Since you are unlikely to have to drop Contador on a mountain stage anytime soon, a few grams won't matter in any way whatsoever. The best thing you can do for yourself as a cyclist is to inoculate yourself against upgrade-itis.
I've owned a bike with 105. Mid-range componentry does everything a person really needs as well as the more esoteric stuff.
Hey, I can't claim I haven't been there, in my more credulous and youthful days. That's how I know it makes no difference, except to whatever else relies on you having money in your bank account.