The terrible problem with space flight is weight. Weight, weight, weight. It takes an enormous amount of fuel and rocket to push any kind of weight into orbit.
But the shuttle is not optimized for getting useful weight into orbit (i.e. payload, with the emphasis on pay). It's optimized for returning the rocket to earth. I.e. it has landing gear, wings, rudder, etc. All this is heavy, and therefore terribly expensive.
Step back a moment, and look at what we really need to bring back from orbit - it's the astronauts. Nothing else. All we actually need is a tiny capsule to bring the astronauts back, i.e. a heat shield and parachute. The rest can just be left in orbit.
Getting rid of all that weight then means the rocket can either be far cheaper, or it can launch much bigger PAY loads.
As for reliability, likely the most cost effective solution is to build a smaller, highly reliable rocket just for pushing astronauts up into orbit. Use another rocket, a big, heavy, unmanned one, and one that needn't be so reliable, to push up the stuff the astronauts need to do their mission.