Honda S2000 = expensive parts, highly stressed engine worth probably £2000-4000? You can probably pick up a used 1.6 or 1.8 for an MX5 for about 500£ or less? And a cheaper gearbox too....
The gearbox is bulletproof, it's not going to go wrong unless you abuse it. Quite why the cost of an S2000 gearbox is relevant to the comparison I don't know.
What do you mean by "highly stressed"? The engine, brand new will set you back a few notes but it's not like they have a shelf life. They are fit for purpose and not what I would call 'stressed'. You can FI a stock f20c and it will be quite happy with it, in stock form, maintained and not abused it's not going to need replacing or rebuilding
just because.
It's a high revving engine with the engineering to let it run like that all day long (Fiber Reinforced Metal liners for example). It's not stressed.
Some parts can be quite expensive and I have no doubt on the whole the MX5 can have more overall parts bought for less. That's not to say ALL S2000 parts are prohibitively expensive like some premium brands, they just aren't.
You will never run an S2000 on the budget you can an MX5, but that is more a testament to how cheaply you can run an MX5, not how expensive an S2000 is.
I ran a Ford Focus for 5 years, the S2000 has not cost me any-more to maintain other than tyres. Servicing is no more expensive than any A to B car, nothing fancy to go wrong electronically, there are a few bugbears that can creep up on you if you do not get ontop of them (Bushes and adjustment bolts for example). I'm not sure where the perception that an S2000 is closer to supercar running costs than run-about costs comes from.
Just because you can spend 5 grand on a Mugen hard-top or a cool half million on a Spoon washer fluid filler cap sock does not mean all parts costs this much