Well, if we entertain the idea that the average price for a LEGO piece is $0.10, and the average LEGO piece weighs 3 grams, then 70 pounds would be...
$0.10 = 3 grams
70 pounds = 31,751 grams
31,751/3 = 10,583
$1,058.30 = 70 pounds.
Over a thousand bucks! Wow! If my calculations are correct, then this would probably be a good deal no matter how much you get it for.
Another fact you must consider though, is that who know what kind of sets he has in there! If it was bunch of Jack Stone sets, it would be worth dirt, while if it was early 2000s Star Wars, you've hit the jackpot! Theres no accurate way to determine the value without going through each individual set and seeing the going price for it on Bricklink...
Overall, my advice is: Go for it! But if you don't have all the dough, but some at a time like you mentioned.
Good luck and happy bricking!
~Flare