I you compare the Michelin Pilot Sport 2 against their Pilot Sport Zero pressure there is nothing in it, around 5 euro a tyre at cost price, same with the Bridgestone and the Pirelli.
Compare that with a full size spare and jack etc. it did save money.
At the end of the day you have been lucky mixing tyres front and rear, occasionally you can get a combo that don't fuck the cars handling up, but more time than not BMWs do not like it and become very odd in teh handling department.
I have had it on 3 different BMWs with mixed tyres front and rear, and then had it with one where it felt fine, and that was an M3.
Until you have experienced just how bad it can be I don't think you can comment on it. I know I thought it was all crap until I had the issue. And in my case they swapped out the whole of the suspension, including wishbones, rear trailing arms, struts, springs and eventually all the bushes, then someone who races CSLs took my car down the road and after the second corner said "You've got different tyres on front and rear!!!"
I was adamant I didn't have, as they were both Pilot Sports, however we jumped out and sure enough he was right, Pilot Sport 1s on the front and 2's on the rear, one is symmetrical and the other asymmetrical, we put PS2's on the rear and bingo, the car felt perfect.
I then had run flats on the front of my E91 for a few days as the front tyres that arrived were the wrong load rating and they had already done the rears and that was the same, no where near as bad but I knew it was not right as the traction light kept flashing and I had never seen it before in that car.
Then on my second M3 I bought some 18" wheels for the track, they had new PS2s on the rear and Kumho KU31s on the front, I had ordered some PS2s for the front before the wheels arrived, but put the wheels on before taking the car into to have the front tyres fitted, and it felt fine, so much so that if I hadn't have already bought the front PS2s I probably wouldn't have bothered.