Going to chime in as I've owned both and I'm with PMKeates. Nothing catastrophic has gone wrong on either car, but the BMW hasn't cost £lol more to run than the Mazda did, don't get me wrong it is more expensive, but insignificantly so. When I bought the 330 my dad did a few sharp intakes of breath through teeth saying how it's going to cost loads, and it just hasn't.
Any work that has had to be done would have ended up needing doing on the Mazda anyway as they're consumables, and yes it has cost a little bit more than it would on the Mazda, but that's kind of the trade off for having 80 more horses under your right foot and a better car. Tyres are £40 more for the rears on the BMW and the fronts are practically the same price, pads and discs are again roughly £80 more all round (parts cost) on the BMW, tax is £20 more on the BMW. All these costs aren't too frequent and are also negligibly larger given how often they need to be paid.
Even fuel isn't significantly different if you're not doing stupid miles a year, the 90 mile commute (there and back) to visit my GF at uni costs £3 more in the BMW than the Mazda if driving normally, £7 if I'm having fun.