And there you have hit at the heart of what you're asking. you have one of the best designed cars on the road and BMW engineers make it a point to squeeze every last bit of power out of these engines they can get. Upping the power from stock requires something serious. You can chip the computer, but aside from that you're left with turbo, supercharging and or nitrous. As far as the cost is concerned, modified cars don't have much of an aftermarket anyway. How many times have you seen some "dumb kid" trying to sell an '88 honda civic for 20,000 because of all the stuff he's put on/in it? Would you buy it? And the same principle applies here. You mod it, you OWN it because 99.9% of the time you'll never resell it for book and even if you get book you're still gonna take a huge loss on it. I'm not saying don't do it, I'm just saying go into it understanding that you will most likely never recupe your money and whatever you do you'd better be happy with and plan on driving it to its death. Some guys will break even, maybe even make a bit. But most of the time that's not reality. If this is your daily driver and only vehicle, I wouldn't mess with it. If it's a toy you want to make a go faster track day car, make a budget and plan on not ever getting any of it back aside from the driving enjoyment it will give you.