I have a 430CUP and a 675LT.
To answer your first question my 430CUP has cost me nothing more than services, around £300pa, and that's it. I did buy a warranty this time as I thought my cats had gone, and they had, so that was £800. But that's all I have spent on my 430CUP in 3 years.
The 675LT is exactly the same, owned 2.5 years and its cost me £1k pa for a service and £5k pa for a warranty. I have needed nothing else in 8k miles.
So in terms of running costs you are really looking at the warranty for a McLaren. Around £3kpa for a sports series and £5kpa for a super series. So the warranty cost is the only real difference. However there are a few good independents now and reliability on the McLarens has settled down. I have only had a broken coolant hose on mine. So you can use 'V' Engineering and probably average £2-3kpa. I only have a warranty so I sleep easy but might pass next time (I just bought 2 years for £9,600)
If you can budget for the warranty or an extra £2-3k pa over and above a 430CUP into a slush fund that should do it.
In terms of driving my 675LT is from another universe compared to the 430CUP and many other cars, its is so quick, totally planted & stable with very good interior quality. The 430CUP would not know which way it went. However that's not the full story, the Exige has the manual box and offers equal levels of engagement and even more reward for less than half the cost. They feel similar in ethos but on the road are very different. They compliment each other very well but are not competitors. That's why I can justify both and adore them equally,
To answer your second question there are 3 types of supercar owners. The ones who want the latest toy and own for 12 months. The ones who want to 'tick a box' and own one once in their life for a year or two and The ones who are enthusiasts and generally buy the cars they like and keep them long term.
There are plenty of McLaren owners in the enthusiast camp but by the time they have bought them they have usually been through the initial trinket owner, the second trinket owner and then a tick box owner. So by the time the first enthusiast buys one they are around 3-5 years old with as many owners. This is standard across most supercar brands. Lotus are mostly enthusiast cars from day one, you have to be a single minded fan to buy one, its not a golf club / spa hotel / Sloan street cruiser.
If you have the additional £3k pa to run a McLaren on top of the outlay then you won't get a faster or more rewarding car but if you can't justify the extra expenditure over the Exige then I wouldn't be too despondent and just revel in the glory that is the Exige.