Many M140i and M240i move up to a true M car and then move back, especially as a daily. Running costs mostly.
The b58 engine is crazy efficient. I've averaged a true 35mpg over 65k miles.
Also to date with service packs ect it has cost me only £600 in total for two oil services, two brake fluid changes, and a big major service. Only other expense is a replacement set of Michelin super sport tyres. So a grand in total for over 65k miles.
I must add though, that I'm at a turning point.
I do need shortly a full set of discs and pads, another big major service, and set of tyres. So do I chop it in, or spend £2k, twice as much as the last nearly 5 years.