Why do you want Run-Flats when you have a COOPER with a jack and a spare? It makes no sense to me. 17" Run-Flats are going to cost $300+ a copy to replace. A good set of non run-flat 17's are $150 a copy. So why get run-flats
MINI is the only way to get the R90 wheels. BBS does not sell them through any other channels.
Here's the thing. Since you have a Cooper you can buy the R90's by themselves ($1551), then get 4 regular non runflat tires ($600), then sell you stock wheels and tires (-$500). So your total out of pocket would be more like $1600. Not great, but not bad either.