Blammer:
Have you thought about seeing if there are 15 people on the list that would prefer to get a brass mould? In another GB I'm in (the 311440) that was done, and the original thought was that they wouldn't get 15, and it's already up to 18 or 19! Personally I would much prefer to have a 4c brass mould rather than a 5c aluminum one even with the higher price. I figure it can't hurt to ask, you might be surprised by the number of folks that would prefer a brass mould.
I don't know if it's just me, or what, but I find that the difference between the 2 materials is much more apparent with a smaller diameter boolit than it is with a large one. With a 30 cal mould the brass holds the heat much better than aluminum, with a 45 cal mould it's not as noticeable.