The recent increase in power and the Spider’s increase in weight has made no difference to the 12C’s consumption or emissions, so in terms of fuel and tax you'll pay a similar amount.
Our own fuel consumption figures were broadly the same – we returned 23.4mpg on a cruise and 19.0 overall.
With a track consumption of 6mpg suggesting it’s best to fill the 72 litre tank before you start a track day, because it’d be dry in about 90 miles.
Other costs are par for the course: the 12C is an expensive car, as are its peers, and that’s that.
Brakes, tyres, servicing, spares and maintenance will all be costly - even more so if you make full use of the performance on offer.
The McLaren's residuals should hold up well though, provided you take care of it and maintain it to the highest standard, as it is a fairly rare car.