To be honest. you should shop for mid priced componants as your low grade parts. The cheaper you go the more you sacrifice in materials & workmanship quality. Unless you are making a batch of about 100 low budget-entry level guitars for a distributer or for a few stores, stay away from realy cheap parts.
Screws can be gotten by the box of 100-500 in an engineering or tool suppliers. They wont be cheap crappy screws either, & will cost next to nothing for the quantity.
A local fabricator could make a batch of truss rods for you at reanonable prices.
Or just buy lenghts of stainless rod & make them yourself. Single action rods end up costing me about €4.00 to make. Double action about €6.00. But I make them in batches of 50-60.
As to ferrules, those are a direct link from the string to the timber. So they can have an effect on tone. If you put in crap grade cast zinc as oppsed to milled aliminium or brass to save $6.00 or less for a build - your priorities are a little out of whack.
Strap buttons & vol knobs, OK. buy cheap if you must. But even these only save $5.00 or so on a build. I see no point.
If you want to save money then buy in bulk. & buy medium to high quality parts. Or even make them yourself if you can.
Buy your timber an large boards - unfinished. for bodies, tops, fret boards. Same goes for fret wire.
Get your horn, bone, hoof from a slaughter house & shell from the beach or a seafood farm (oyster, clam, abalone farm)
But realy. Saving are only realy viable over a number of builds, Say 20 or more.
& ask yourself. If you commosioned a custom guitar & the builder used budget grade parts instead of the very best your budget allowed, would you be happy ? Would you ever send a friend to that builder ?
I know its a rant. But seriously man. For a saving of maby $50.00 its just not worth all the hassle