You'll never go back to pontaic if you can suck up the fact the bmw might require a little more attention...but I'm sure you will when you quickly become addicted to the car.
If you take it to a independent BMW or german mechanic (who I would take my car to if I didn't do my own work), costs shouldn't be that bad.
Though you should be asking the 525i and 530i owners - the 530i is an inline six which is another one of bmw's bulletproof motors. The 540i is a v8 and costs more to maintain. You should just be prepared to put a little more money into the car than you did with the pontaic. My dad has a 1999 grand prix and has put maybe $400 of maintainence in it the past year, so they are very reliable cars...if it were for sparkplugs and oil, you could weld the hood shut and drive the car till it dies (becuase by the time it needs suspension and other stuff, it wont be worth more than $2k).
I think owning a bmw, you have to take on the mindset that you're buying a car for more than just transportation. You have to WANT a car that is world-renowned for its overall package (handling, comfort, brakes, etc.) You will pay more to own the car, but you should have MUCH more fun driving it - and you have to KNOW that you, deep down, are willing to pay a little more each year to own the type of vehicle that CAN give you a good joy in driving.
My dad's car isn't a bad car - but the brakes are mushy, the suspension is mushy, it moves decent but everything is so plush and mushy, and the steering is way overboosted as compared to every foreign car I've ever driven. My 1989 535i is still much more solid. The biggest thing between ALL other cars and the BMW's I've owned and driven, is that the BMW's give you a very confident feeling when driving them. Sure the grand prix is an EASY car to drive, but there's always little bit of touchyness in the steering and brakes...you just don't feel like the car will always move and always stop when and HOW you want it to. I say go ahead and get a 530i, much cheaper to own, MUCH cheaper with gas costs, and definitely no lump when it comes to speed..you DO have 220 hp with that, and the inline 6 is absolutely the smoothest motor around.
Hahah long post, hope it provides a little indirect insight...good luck!