I cannot see paying much of a premium for a pre-ban AR-15. You can get an excellent new (with full factory warranty) post-ban AR15 fitted with a muzzle break that even looks like a pre-ban for $750 or so, unless you want the rail mount handgaurds, etc. If you want to go to the trouble (or fun) of building your own AR you can save perhaps a hundred bucks from that.
I do have two pre-bans, but got them long ago, my post bans are every bit as good except you cannot fit a colapsible stock. It is not like the post bans are made of cardboard or something horrible.
If I were looking for my first AR15 it would be a new post-ban unless I could easily afford the extra six to eight huindred bucks.
And don't slam the AK-47, AK74's too bad, they are reliable as hell and inexpensive too. (But not like you suggest, precisely fitted like the AR15). You can get a nice custom built and refinished AK for $600, and entry level new AK's start below $300. I have a restocked AK74 (5.45 x 39 caliber) that I enjoy, got about $400 into it total.
Will a pre-ban AR15 that you buy today for $1600 be a good investment? Who knows. The investment angle is another thing alltogether, but I'm betting you want an AR15 to shoot. Remember a top dollar investment gun must remain unfired to stay in the top price bracket!