This sounds "fishy" to me. Are you sure you are not trying to find out how many ballons attached to your lawn chair you will need to fly up to 30.000 feet lol !
But, I usually have large helium ballons flying over my shop when i have a ride. I got 3, 6 foot diameter ballons for $60.00.
A large tank of helium cost me $180.00. That is just enough to fill 3, 6 foot ballons. I think i read that one 6 foot balloon will lift 6 lbs. But i may be wrong on that? And that doesn`t mean 1, 1 foot balloon will lift one pound. But i did put a small video camera on one 6 foot balloon and it lifted it with no problem. I also put the same camera in my RC airplane and flew it around and got some good video of the riders lined up before the start of the ride.
I did check out blimps at the same time, but they are too expensieve. They go from $500.00 to $30,000.00 !!