Acyclovir is still way cheaper but if you have
insurance coverage that makes generics low cost,
valtrex is, as of this month, going to be WAY cheaper for you now :)
the technicality to the answer to your question is this - only valtrex has been studied for reduction in transmission to a partner. We'll never see studies with famvir or acyclovir either since both are generics and no money is to be made from doing the studies at this point.