As I understand it, the maps are generally available they won't work on your car unless you have a VIN-specific unlock code. The scheme to create the unlock code has been hacked and there was an official vendor on NAM selling update maps at low prices. This vendor kept a low profile with no website, only a gmail address. However, I see that the vendor is no longer there (probably taken down by BMW).
So to answer your question, yes there are cheaper sources for maps out there but you may have to find them on the dark web and give payment in bitcoins...
