There's a couple of threads along these lines at the moment.
My experience is similar to many on here. I bought pretty cheaply at the start of 2015, getting an early 3.4, 65k miles, pretty good history, all original with the MO30, LSD etc for £9,800. I had a PPI and new it would need a few grand to bring it up to scratch.
I haven't actually added up everything I've spent but it will be comfortably more than I paid for it.
Aside from the fact that I'm skint, I don't have a huge problem with that. it's a 20yr old car that cost close to £60k new.
I have read numerous times that just because £60k car has devalued to £10k, for example, it is still a £60k car and the running costs will be in line with that even if they don't need major work doing. As many have said these cars are old now and need a lot more than just routine maintenance.
I think the problem comes when people buy a £10-£15k car expecting it to cost fiesta money to run.
I'm quite comfortable with the risk, I am driving a car way above my pay scale and even if it went bang tomorrow and I had to sell for parts, I would have owned a 911, which for me is a tick in a box I never thought I'd have.
In reality it is not my only car so if the worst happened, I would put it away in the garage until a time came when I could fix it.
As people have said, everyone has a different attitude to risk but I think you have to remember you cant buy a £10-15k Porsche and expect it to cost nothing run. Someone will be along in a minute to say they are doing just that, but I expect that's the minority.
Anyway, buy them, drive them and enjoy them. If the risk is too great to sleep at night then buy something cheaper!, nothing wrong with an MX5 by the way!