The only big cost of doing twins instead of single is that you end up with 2 waste gates. Other then that, it's pretty similar in costs. You have to build two hot sides anyway. You either merge the exhaust, or you merge the boost. Either way it's about the same fab work.

With twins you'll need two WG's though, so that is about your only redundant cost. In general two small turbos = $$$ 1 big turbo. The packaging of twins is definately easier since you have a much smaller turbo to work with. Packaging a big T6 can be a bit of a hassle, but nothing impossible.