The drivetrain is likely the Spectre's, but the body is custom. Performance/technical engineering isn't the only reason to pay a lot for a limited run car.

The low production number already means the price will be high. Since cost constraints are lower, designers usually take the opportunity to exercise more creative freedom which often leads to more difficult manufacturing or special parts. That adds more to the price.

Then a lot of what that kind of client wants is handwork, which takes a very long time. The only people allowed to do it are very skilled and command a high hourly rate, which again increases the price.

On top of that the buyers want special and unique materials. Sourcing them and testing them for automotive suitability is expensive. RR will have bought and thrown away a lot of very expensive cashmere for the convertible top. Add more to the price.

By the time you do all that, a 7 figure price isn't unreasonable. Whether it's unreasonable to make a car that way in the first place is a different question. But if you take that as the starting point, the sum of all those expenses is higher than you might think. Of course there's much, much higher margin for these cars than anything else. But it's not like this car costs RR the same as a Spectre and everything else is markup.