I paid a lot for these seats---$1600. Yes, ouch. You might do better, though. I just couldn't find any undamaged ones for less. Once I sat in them, though, the pain of spending that much quickly faded away...
For those that are interested, here are my DIY instructions for putting them in:
In the 3-series, the seatbelt attaches to the rear of the outboard rail, while in the M3 it bolts to a totally separate bracket. So the main problem is how to attach the seatbelts. The answer is to swap the seat rails.
First, obtain M3 seats. Be aware that the leather is slightly different. If you have a gray interior, the M3 seats will be slightly darker, but the difference is much less noticeable once they're in the car. I can't vouch for other colors. My swap was an '01 M3 to a '02 330i.
Also, I carefully checked the wiring harnesses on both cars before buying my seats to make sure they had equivalent pin configurations. I'd advise you do that too.
To install them, you first have to take the original seat out: first, tip the seatback as far forward as it will go (for ease of removal). Then, raise the whole seat up a few inches from its bottommost position and remove the seatbelt. Run the seat backwards and remove the front bolts, then run it forward to remove the rear bolts. Doing it this way leaves the seat in a configuration that won't tip over backwards once you set it on the floor. Lastly, unplug the harness and lift the seat straight out. I put old blankets down around the seat, over the carpet and doorsills to prevent grease marks and possible scratches.
Once the seat is out, I balanced it (at the seat crotch) upside down over a well-padded sawhorse so I could work on the bottom (take the headrest out first so it doesn't rub the floor). Do the same with the M3 seat.
You'll see that both seats are attached to their respective rails by four legs that provide seat height adjustment. These are attached with four nuts and spring clips, which are easily removed, allowing you to swap rails between the M3 seat and your own---with a couple of complications:
First, you have to unbolt the female part of the seatbelt from the seatrail, and unclip its wiring harness from the bracket. You also have to remove the mounting bracket for the other harnesses and detach the one motor from its connector. At that point, you'll be ready to remove the 4 bolts and then the rails, one side at at time.
BE CAREFUL HERE! The M3 seat gave me no problems, but on the 3-series seat, the front legs are spring loaded (serious springs), and once you get the second rail off, those front legs will snap forward hard enough to break a finger. Be aware of that, and don't let a finger get in the way.
Now you simply install the 3-series rails on the M3 seat. This is a relatively minor exercise in getting things re-aligned and bolting them back together. I also installed the M3 rails on the 3-series seat, mostly so I wouldn't lose parts, and so I could store the 3-series seats more easily. Be forewarned, this is much harder because of those spring-loaded front legs, but the good news is, it's also optional!
Once your M3 seat is assembled with 3-series rails, installation is as easy as lifting them back in. Position the seat, plug it back in, run it forward (if needed) to install the back bolts, then run it back to do the front bolts.
At this point, you'll find you can't get to the front bolt. Remove the plastic seat-rail cover and you'll be able to reach them (it unsnaps at the back first, then lifts off). Then, re-install the plastic cover, attach your seatbelt, and repeat everything on the other side.