Seeing as I use a 70" screen, I'm betting it's more then 20-30lbs more...if there's even a 70" plasma on the market right now.
I compared the weight of my Panasonic 65VT60 and googled a Samsung 65" LED.
That was the difference. Other LED's may vary.
There is no 70" Plasma. Don't think there's anything bigger than a 65"(actually, I guess it's 64" now that Panny quit making them) that you can realistically purchase.