it is the design as well as the diameter of the piping... generally the diameters don't vary too much... so then it leaves the design to be best..

one thing that will always determine the cost..is whose name is stamped on it.

as far as the design... a good flowing tubular manifold will flow much better than a weak rough cast manifold... the way air travels through tubes is spinning along the walls. so if you were to make a rough cast style manifold... the walls aren't as smooth, the curves are less fluid.

so to answer your question simply... yes the design matters... thats how companies like TODA can make a race header that will make 15hp at the wheels when coupled with toda exhaust... and DC using SIMILAR (but not exact) design will only make 2-3hp tops....

in short... u get what you pay for