Well... Normal scheduled maintenance isnt all that bad... Probably pretty similar to the rx8. Where it will get stupid expensive is if things break. Turbos are somewhat common issues on the 07's so that could be potential prooblem at some point. Also minor stuff is just really expensive, especially if you go to the dealer. My advice would be to learn to work on it yourself and save TONS. Comparing costs toan RX8.... Considering rx8's tend to need motors pretty often and burn more fuel... I'd say long term the 335 might cost less even. The rx8 has a ton of common problems that will likely re occur... BMW at least revises parts so you probably won't have to replace the same part twice.
Regarding what it costs... Oil change is scheduled every 15k miles or so and usually around 100 bux from the dealer. Some people recommend doing it more often. I personally think 7k - 10k is good.
Every two years brake fluid should be flushed 100 bux or so.
Cabin filter is 50 and is so easy you should DIY
Battery should last 4 or 5 years but is about 300 or for the dealer to replace it. The car does need to be "told" when the battery is replaced. You can buy the cable and software or you can pay the dealer or a shop that has it. Just make sure it gets done.
Air filter not sure of cost... But again DIY can't be more than 50
Transmission fluid should be replaced probably every 50 or 60k even though BMW says it's life time fluid. Auto is 300 - 400 to replace. Manual is probably 150 ish depending on what replacement fluid you use.
Diff fluid can probably go 100k miles... And should be similar in costs to the manual trans change... Maybe a little less.
Coolant is best replaced every 2 years... Not to exceed 5 years (IMHO) Dealer is probably a 150 - 200... Guessing. If you DIY it's a 20 dollar jug of BMW blue coolant distiller water and an hour or two of your time.
Power steering is a good idea to have replaced in the 60k or so range... Again probably 150 bux guessing.
Like I said overall as long as nothing breaks its probably similar or better than the rx8... However some people have lots of issues and others don't.... Just depends on your luck