I make a lot of my own gear, like others in this thread, i suppose. i admit that sometimes, when im done wiht a project, i have to take a deep breath and really understand that there is something to be said for having made it yourself. if you consider your time worth money (lets say i think my time is worth 12 dollars an hour..) then some of the thigns ive made have been pretty expensive.
i actually disagree with what people say about materials costs. i think materials costs are generally really low. really really low. and that doenst assume non-domestic rip off cheapo fabrics. also- if you cant find some elusive incredible buckle duraflex makes, sometimes a local diostributor will just sell you one or two- as a sample. or just beucase they're nice. theyve done this for me.
ive made all kinds of junk. some of the more complex stuff im glad i did, but will never do again. its up to you.
im not sure what you would wnat to seam tape on a pack. ive never seen a pack seam taped, unless you want to make some kind of dry bag. seam taping is definitely doable at home, although is much easier with 3-ply (fabric lined) waterproof fabrics, instead of fabrics with the laminate or coating exposed.