Last one I bought from a scrapyard (See brianmoooore's E30 related holiday problems thread) cost me a fiver, but that was from an exceptionally cheap scrapyard.
On 318i's, at least, the changeover from twin to single pump was sometime in 1987. I had two '87 318s, and they were one of each.
Some E30 fuel pumps have one rubber fuel pipe connected to them, others have two. If your existing pump has two pipes, then your replacement must as well, although the actual pump from a single pipe can be fitted to a two pipe mounting frame with a little work.
If you use a two pipe pump to replace a single pipe pump, then the extra pipe needs to be blanked off with a short piece of hose and a plug.