I just replaced the top on my 97 318 and I bought an aftermarket top on eBay for about 300. The website of the company I ended up getting it from was www.topsonline.com
The top is very good quality, but it does not really compare to the original. It is much louder and does not come with a zip out rear window, but is worth the savings if money is tight. I was able to get it replaced for about $2,000 less than what the dealer quoted me.