It depends......NASM holds the majority of the drawings form WW2 and earlier. They have a prohibition from using them to construct a flying aircraft from the drawing sets. You must sign waivers and a use agreement prior to having copies made. As to cost, it depends on the number of rolls and what grade duplication you order. I believe current cost for a diazo duplicate is about $40 a reel. The AT was 9 reels and I think the B-52 is something like 200 reels. The Navy holds the drawing sets for many aircraft, but I don't know the current availability or cost for them, or the current contact information.
Depending on what you are interested in, some common aircraft are cheaper to purchase than to scratch build. For something that no longer exists or is unobtainable, then scratch is the only way. If you haven't built an aircraft before, or are not familiar with aircraft production, then it will be a sticker shock at what it costs to build. If you plan on building anything post about 1920, you will need to factor in setting upyour own machining capability, due to very small number of machine shops that will take on one-off aircraft work. For a reference point, you can guestimate 3-400$ per cubic inch as a minimum for fittings at a commercial shop. It's not for the faint of heart or light wallets.
