AR's start cheap. You can build them cheap. The trigger and barrel are the soul, don't go cheap!
This is my first post here, but I have thousands on AR15.com and have built a couple AR15s.
This man speaks the truth.
Places you can cut corners/costs:
- stock (the regular A2 stock is cheap and easy to find used. And it doesn't cost that much more to get a simple adjustable stock from one of the zillion people making them). No reason to drop $250 on a Magpul PRS right off the bat.
- grip. Go with a $5 A2 grip to start with. You can step up to a $30-$60 grip later. There are advantages to having a more modern grip angle or a higher grip on the gun, but for static shooting to start with, the grip isn't the most important piece.
- free float tube. The simple ones are simple and cheap. The quad rails can get very expensive. Granted, they look cool. But it doesn't sound like you have much use for one and, again, they can be added later.
- stripped lower receiver. This one is much debated. One argument is that they are all basically the same, and it basically just holds the trigger group and magazine (so as long as it is machined roughly to spec and can be assembled, it's fine). Other folks pay premium bucks for stripper lowers. I think most people will agree that a quality upper is more 'valuable' on a precision rifle (because that's where the bolt carrier group rides) than a quality lower.
Places where you have to pay to play
Again, agreeing with the guys above: don't cut corners with the barrel or the trigger.
If you've mostly shot tuned bolt actions, you will be in for a rude awakening shooting a 'stock' AR15 lower like a Palmetto Arms lower with the regular Lowers Parts Kit (PSA will sell you a LPK less the trigger, for this exact reason).
The standard AR trigger is a two-stage, military trigger, probably a 7-8 lbs pull. The most common aftermarket triggers are basically the same style, but with different springs. They are modeled after the National Match triggers, legal for DCM/CMP competitions, and are two-stage triggers that have pull weights of about 4.5 lbs. These are also popular with the guys doing 3-gun competitions who are 'running and gunning' with their ARs and shooting on the move. You can get one of these triggers in the $89-$120 range usually.
If you have been shooting a nice Savage or Rem 700 bolt gun,
you will be disappointed with it. After the first range trip, you'll want something approaching a bolt gun feel (single stage, 2-4 lbs pull). BobD above is 100% right: the Geissele Hi-Speed Match is basically the most expensive AR15 trigger commonly available, but it's also the one that's going to have the characteristics that are best for precision shooting. There are a handful of other match or varmint type triggers; all of them over $200 and many with questionable reviews.
Regarding a scope; you know what scopes you have used in the past and how they have worked for you. Try to mount the scope so that it's 2.6" above the bore (which is not hard, as a great many AR15 mounts allow for this).
Rough Estimate
Palmetto State Armory (or similar) assembled lower: $200 (or you could buy a LPK, a stripped lower, and a stock and put them together but it would still cost about $200). You will have to buy this at a gun store and have an FFL transfer (unless it's a private party, face to face sale).
Match-type single stage trigger (Geissele, Chip McCormick, Jard, Jewel, etc): $200-280
So we're at basically $400-500 for the bottom half of the rifle.
Uppers are different. They can be mail ordered and shipped right to your door. No FFL on either end required.
Because of the nature of AR15s, there are
a lot of very lightly used uppers for sale on the various forums. If you're patient, I'm sure you can find a lightly used bull barrel upper for $500. You'd be surprised (or maybe you wouldn't be) at the number of people that buy varmint or match barrels and then within 25-200 shots, realize it isn't for them, they don't have time, it's not as fun as blasting at dirt clods with Russian steel-case ammo, etc.
But before buying a barrel,
figure out what grain bullets you want to use. Do you want to use the commonly-available hunting and varmint rounds? If so, a 1/9" twist barrel for the 50-60 grain rounds is probably where you want to be. The 1/7 and 1/8" twist barrels are better for the heavier, longer, 77 grain match ammo which is popular (arguably,
required) at the 600 yard line in DCM/CMP competition.
White Oak Armory makes great competition rifles. They also build varmint uppers for $610-650
White Oak Armament | Complete Uppers | Varmint Uppers
The various big names (Bushmaster, Armalite, etc) all have a varmint model. You will see these uppers for sale, either new or used. A new Armalite upper would also be about $600.