at 24mpg on unleaded, i'd expect 20 - 21 on LPG, so yes, its dramaticly cheaper.
Alternative automotive fuels in woburn sands are excelent, they did the (admitedly state of the art) conversion on dads tahoe, you do pay for top notch kit (£2700 for the prins injection system) but there is NO power loss at all, its impossible to tell what fuel the car is running on, except for the cost.
i'd expect the Z28 to be a bitch of a job, so i'd steer way clear of anyone who claims that they can do a top notch job for £1500, and avoid anaything that is not sequential injection..
also, the vehicle needs to be running spot on for LPG - we had issues with a slight stiumble and miss on light throttle, and simply couldn't pinpoint it, turned out the inlet manifold was cracked in 3 places!