In regards to the original question of this post.
Put simply HP = torque multiplied by R.P.M. / 5252.
Therefore to increase the horsepower of your engine your either need to increase its torque or increase its R.P.M. or both.
Forced induction (Turbo's, superchargers or Nitrous) creates H.P. by increasing the torque.
Thoretically increasing the displacement of the engine.
Hence the aphorism "There is no replacement for displacement".
Yes there is and that's R.P.M.
To increase the R.P.M. of an engine (reliably) and get it to start making power higher in the rev range there is a lot more work needed to be done.
Which is were the extra cost is.
Dollar per H.P. F.I. will always be cheaper than N.A.
However your choice on which route you chose to follow should come down to what you want your car to do and why you bought it.
The power curve of a Naturally Aspirated engine is much more linear than that of a large Turbocharged vehicle hence why N.A. cars are better suited for circuit/track racing.
If you have ever tried to push a rear/front wheel drive car with a large turbo charger fast around corners you will know what i mean.
As McChook and DynoDave previously stated it is also important to build a complete package i.e. engine/transmission/sussy e.t.c.
It is no good having all the power in the world if you can't get it to the ground and keep it to the grouund through corners.
There is more to driving than straight lines, and most nice roads i know off go for longer than 400 metres.
On the subject of efficiency Honda produes THE most efficient engines in the world.
They are also the single largest manufacturer of Internal Combustion engines in the world.
As for how far you can go with N/A......
All the way, here's an example.
http://www.ericksracing.com/racecar.htm
Regards,
James.
