There was at least one Shuttle flight planned to test the CRV in space. Those cost, depending on whose numbers you choose to believe, anywhere from $400 million to $1 billion. If the full cost was a part of the CRV program, well, then comparing it to the cost of developing the 747 (back in 1969, adjust for inflation) isn't exactly fair. From what I read, the CRV program was intended to build several orbital vehicles.

Going from a CRV to a fully functional orbiter would certainly be possible, but it would require many changes. For example, you'd need to upgrade the avionics to provide for orbital operations such as rendevous. The CRV only needs to know how to deorbit and land. You'd also need a substancial upgrade to the vehicle's propulsion system. The CRV was planned to only have a deorbit engine (possibly solid fueled) and used pressurized nitrogen for the attitude control thrusters. That wouldn't be sufficient for full orbital operations. If you plan to launch it on expendables and carry humans, you'd need to man-rate the booster. That's also a nontrivial task.

I believe there can be some commonality of design between a simple CRV and a fully functional reusable orbiter. I'd prefer the orbiter be big enough to carry a reasonable crew (say 6 or 7) along with a few thousand pounds of supplies to the ISS. Bigger payloads can be launched on unmanned boosters.