The Roton might have worked for that. I worked on many other designs like the Phoenix and similar concepts that would have done that. There are designs for spaceplanes that launch from the back of an airplane that are even more feasible, although you might get an argument about the definition of SSTO on those.

Costs? Haven't priced it in a long time, but everything I worked on had development costs well under the billion dollar mark back then.

Most current private-sector designs are standard manned space capsules of one kind or another on conventional staged boosters, although most of the booster designers are either specifying, or working toward, booster reusability.