The ever-readable Rocket Man has a post by reader Kelly Starks that has some interesting numbers regarding the cost of SSTO (Single Stage To Orbit) and TSTO (Twin Stage To Orbit). Of particular interest is DARPA's concept of using upstream water-injection cooling, and LOX (Liquid Oxygen)-injection on conventional military Turbofans to reach Mach 6.
I've been interested in such Hybrid systems since I took a supposedly reasonably accurate SR-71 Flight Simulator for a spin (literally) and had it zoom to 160,000 ft, after a Mach 3+ run at 80,000 ft followed by a shallow dive and pull-up when the air got thick enough to give some bite for a controlled entry into a climb. Of course I wasn't able to recover controlled flight afterwards, and in the simulation the airframe broke up on what was essentially re-entry. But a purpose-designed aircraft just might be able to get a parasite payload up to 200,000 ft on a ballistic trajectory, given enough momentum. The numbers are in the right ball-park.
The great advantage of this is that no new technology is needed. Get an off-the-shelf Russki booster up that high and going that fast, and you start to get really high payload ratios.
Instead of the $10,000 a Kg you get with the very best rockets today, you might get as low as $1,000 a Kg. That's less than 1/20 the cost of a Shuttle.