Depends on the quality of the parts used. Burns Stainless, for example, makes top quality parts and you could spend $440 on a custom tube stainless 2.5 inch X pipe. Or you can spend $85 on a stamped stainless 2.5 x-pipe by Magnaflo at Jegs. Or you can have low paid Asian workers cut inexpensive stainless mandrel bent tubing and weld one up for even less.
I don't need the expensive top quality product for this application. For those who do, it is great there are suppliers. To me the E90M3 is just a daily driver car. For others, it is their most prized possession and it gets a significant amount of their attention and resources. I like reading about the cars owned by those people -- they are often forging the way for modifications.
I will try to get some pictures when I cut the x-pipe for the resonators. I am doing this on jaxkstands so I won't be able to get the best shot. It is very easy to remove and install one of these, even on your back.