I hear what you saying, but is there anything else than an issue of material being burnt in atmosphere due to speed lost/air friction? Gravity will make sure you will get back on Earth so its not like you have to spend another million bucks to get on earth. Given you have a heat/damage-proof can and know when to open parachutes (I know its more complicated than that), you should be good to go (I mean - come back).
I would guess that just getting from the ground into space requires more fuel than traveling by plane 180 degrees around the world (the longest possible great circle route for air travel). And it's not just a maturer of waiting for improved efficiency. Reaching a given altitude will always require energy proportional to mass and desired altitude (until we build a space elevator and can offset that by dropping a counterweight down shaft number 2).
The big fuel tank has significant foam insulation because its two major fuel components are kept very cold (liquid oxygen at -300F/-180C; liquid hydrogen at -420F/-250C) and the external surface gets fairly hot at the speeds the shuttle travels.
Lockheed designed the tank with insulation outside the main shell, and it's never held together very well. I've spoken to retired engineers from another company who designed a competing tank with internal insulation; they're still upset their version didn't get picked. I've never heard a clear explanation as to the specific reasoning for Lockheed or NASA's decisions.
Of particular note, the Columbia disaster was caused by one of those foam pieces breaking off during takeoff and damaging the thermal protective tiles on the shuttle's wing. During re-entry, the edge of the wings reaches a temperature of around 3000 F; damage to the protective tiles basically allowed hot air to burn through the wing.
