I'm pretty sure that either the "PERB" would need to be so massive to get enough of a pressure differential to cause lift that the FAA(USA) or CAA(UK) would require you to jump through very expensive hoops, and also that I struggle to think of a material strong enough to hold an almost-vacuum yet light enough to lift off! Also bear in mind that any payload and recovery system is likely to weigh upwards of a KG...

For my money: just buy a weather baloon for £15 on ebay and get a cheap cylinder of helium from your local welding supplier and you can be on the edge of space within a couple of hours!

We can trade height and mass for velocity - by dropping down things from ballon.

So I wonder if it's possible to attach pulley to ballon with huge lift (according to http://www.chem.hawaii.edu/uham/lift.html helium ballons have almost exactly 1 g of lift for 1 liter, which is 0,001 m^3). So ballon with 20 m of diameter will have over 4 tons of lift.

For 4 tons spherical lead weight with 0.62 m radius and 1.2 m^2 crossection and 0.47 drag coefficient dropped from 20 km this web calculator ( http://www.grc.nasa.gov/WWW/k-12/airplane/termv.html ) returns terminal velocity of 1505.569 m/s which is probably a little too big because it's hipersonic so drag coefficient will increase, but still, it's over 1/8 of escape velocity.

Now to get a very strong, frictionless :) rope. Attach the weight to this rope, put the rope throught pulleys system multiplying the velocity 8 times, and attach the second end of rope to payload (very light - sth like 50 kg for example) lying on ground.

Now fly the ballon to 20 km, leaving payload on the ground, drop the weight down, and look how your payload flies to space.

Will it work? Probab ly not, because of rope-pulley friction..

"I struggle to think of a material strong enough to hold an almost-vacuum yet light enough to lift off!"

Ah, no the whole point of the PERB is that it doesn't need a near vacuum -- just as much partial vacuum as is required to reduce its net mass to zero at the altitude where it currently is.