The autodip is not an issue, you will be replacing just the heated dimming glass element. It pops right out once you can get something behind it to pull on the glass. There is a short pigtail wire electrical connection, but that's it.
As far as finding a replacement for less than BWM dealer prices, try ebay. That's where I found a used one for $115 shipped (from Latvia, probably not with the original owners consent). Salvage yards might be worth some effort as well, although a lot of them are on ebay already. Good luck!
2008 135i Sparkling Graphite Metallic - 6Sp - Black Boston Leather - Gray Poplar trim - Sport Package
Premium Package - Comfort Access - iPod/USB - HD FM - Premium Hi-Fi - Heated Seats
5/27 Euro Delivery Welt - 6/5 Drop off AMS - Re-Delivery 8/8